[yaml2obj] - Allow custom fields for the SHT_UNDEF sections.

[grimar]

Summary

This is a follow-up refactoring patch for recently
introduced functionality which which reduces the code duplication
and also makes possible to redefine all possible fields of
the first SHT_NULL section (previously it was only possible to set
sh_link and sh_size).

[MaskRay]

test/tools/yaml2obj/elf-custom-null-section.yaml

127

Is there a reason for the .foo -> 'foo' change?

SHT_PROGBITS should be aligned

tools/yaml2obj/yaml2elf.cpp

966

What happens if there is non-SHN_UNDEF section with an empty name?

[grimar]

test/tools/yaml2obj/elf-custom-null-section.yaml

127

Is there a reason for the .foo -> 'foo' change?

It's just a different test case. (I added a one or two to the begining, so had to shift down the others).

SHT_PROGBITS should be aligned

I think it is already aligned. We align fields for each section separatelly
taking in account the largest length of the field used.

tools/yaml2obj/yaml2elf.cpp

966

Nothing bad. We have a test case with such situation, btw, it is:
https://github.com/llvm-mirror/llvm/blob/master/test/tools/yaml2obj/unnamed-section.yaml

ELF string table starts from zero byte, i.e. it contains the empty string:
https://github.com/llvm-mirror/llvm/blob/master/lib/MC/StringTableBuilder.cpp#L168
so there is not problems to get the "" string offset.

Initially unnamed-section.yaml seems tested thet we do not crash when have an unnamed section.
When I added the support for zero/undef sections I got complains in this place about "Repeated section name...<empty>"
for unnamed sections. I decided that we can probably just allow them.

[jhenderson]

One nit, otherwise LGTM.

test/tools/yaml2obj/elf-custom-null-section.yaml

131

Should this be %t6, like it was?

[phosek]

The elf-custom-null-section.yaml test is failing on macOS with the following error:

FAIL: LLVM :: tools/yaml2obj/elf-custom-null-section.yaml (32620 of 32657)
******************** TEST 'LLVM :: tools/yaml2obj/elf-custom-null-section.yaml' FAILED ********************
Script:
--
: 'RUN: at line 5';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/yaml2obj --docnum=1 /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml -o /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp1
: 'RUN: at line 6';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/llvm-readelf --sections /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp1 | /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/FileCheck /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml --check-prefix=DEFAULT
: 'RUN: at line 25';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/yaml2obj --docnum=2 /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml -o /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp2
: 'RUN: at line 26';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/llvm-readelf --sections /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp2 | /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/FileCheck /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml --check-prefix=DEFAULT
: 'RUN: at line 47';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/yaml2obj --docnum=3 /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml -o /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp3
: 'RUN: at line 48';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/llvm-readelf --sections /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp3 | /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/FileCheck /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml --check-prefix=REDEFINE
: 'RUN: at line 74';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/yaml2obj --docnum=4 /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml -o /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp4
: 'RUN: at line 75';   stat /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp3 > /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp.txt
: 'RUN: at line 76';   stat /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp4 >> /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp.txt
: 'RUN: at line 77';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/FileCheck /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml --input-file=/b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp.txt --check-prefix=SIZE
: 'RUN: at line 101';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/yaml2obj --docnum=5 /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml -o /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp5
: 'RUN: at line 102';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/llvm-readelf --sections /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp5 | /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/FileCheck /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml --check-prefix=OTHER-SECTION
: 'RUN: at line 131';   not /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/yaml2obj --docnum=6 /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml -o /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp6 2>&1 | /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/FileCheck /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml --check-prefix=CASE4
: 'RUN: at line 147';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/yaml2obj --docnum=7 /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml -o /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp7
: 'RUN: at line 148';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/llvm-readelf --sections /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp7 | /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/FileCheck /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml --check-prefix=DEFAULT
: 'RUN: at line 161';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/yaml2obj --docnum=8 /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml -o /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp8
: 'RUN: at line 162';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/llvm-readelf --sections /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp8 | /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/FileCheck /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml --check-prefix=MULTIPLE
: 'RUN: at line 188';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/yaml2obj --docnum=9 /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml -o /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp9
: 'RUN: at line 189';   /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/llvm-readelf --sections /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp9 | /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/bin/FileCheck /b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml --check-prefix=OVERRIDE
--
Exit Code: 1

Command Output (stderr):
--
/b/s/w/ir/k/llvm-project/llvm/test/tools/yaml2obj/elf-custom-null-section.yaml:79:9: error: SIZE: expected string not found in input
# SIZE: Size: [[FILESIZE:.*]]
        ^
/b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp.txt:1:1: note: scanning from here
16777218 988254754 -rw-r--r-- 1 chrome-bot staff 0 377 "Jul 24 06:00:40 2019" "Jul 24 06:00:39 2019" "Jul 24 06:00:39 2019" "Jul 24 06:00:39 2019" 4096 8 0 /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp3
^
/b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp.txt:1:171: note: possible intended match here
16777218 988254754 -rw-r--r-- 1 chrome-bot staff 0 377 "Jul 24 06:00:40 2019" "Jul 24 06:00:39 2019" "Jul 24 06:00:39 2019" "Jul 24 06:00:39 2019" 4096 8 0 /b/s/w/ir/k/recipe_cleanup/clangKW_Ksk/llvm_build_dir/test/tools/yaml2obj/Output/elf-custom-null-section.yaml.tmp3
                                                                                                                                                                          ^

[jfb]

Reverted in https://reviews.llvm.org/rL366937

[grimar]

So seems stat tool doesn't print "Size: <size>" on MacOS like it does on linux/windows.

Do anyone have an idea how to test that 2 outputs have the same size?

In other our tests we used something like:

# RUN: yaml2obj --docnum=2 %s -o %t2
# RUN: yaml2obj --docnum=3 %s -o %t3
# RUN: od -t x8 -v %t2 > %t.txt
# RUN: od -t x8 -v %t3 >> %t.txt
# RUN: FileCheck %s --input-file=%t.txt --check-prefix=CASE2

# CASE2: [[OFFSET:.*]] fefefefefefefefe
# CASE2: [[FILESIZE:.*]]{{$}}
# CASE2: [[OFFSET]] fefefefefefefefe
# CASE2: [[FILESIZE]]{{$}}

But I found that it is not correct. The value of FILESIZE actually doesn't point to the
last empty line. I tried to use stat because of that instead here.

[grimar]

What about if we add something like "FIle size: <x>" field to the llvm-readobj?
Like:

~/LLVM/build/bin/llvm-readobj test.ooo
File: test.ooo
FIle size: XXX
Format: ELF64-x86-64
Arch: x86_64
AddressSize: 64bit
LoadName: <Not found>

[MaskRay]

In D65140#1600527, @grimar wrote:

What about if we add something like "FIle size: <x>" field to the llvm-readobj?
Like:

~/LLVM/build/bin/llvm-readobj test.ooo
File: test.ooo
FIle size: XXX
Format: ELF64-x86-64
Arch: x86_64
AddressSize: 64bit
LoadName: <Not found>

Ah sorry I missed the use of stat in the patch.

I'm not sure if we want to append FIle size: to the llvm-readobj output - it isn't an ELF field. One can do cp /usr/bin/cat /tmp; echo >> /tmp/cat; /tmp/cat # still a cute cat :)

If you just want to check the size of a file, just use ls -l. As POSIX says:

If the -l option is specified, the following information shall be written for files other than character special and block special files:

"%s %u %s %s %u %s %s\n", <file mode>, <number of links>,
    <owner name>, <group name>, <size>, <date and time>,
    <pathname>

You can just use ls -l %t | cut -d ' ' -f 5 to retrieve the size field.

[grimar]

In D65140#1600570, @MaskRay wrote:

If you just want to check the size of a file, just use ls -l. As POSIX says:

If the -l option is specified, the following information shall be written for files other than character special and block special files:

"%s %u %s %s %u %s %s\n", <file mode>, <number of links>,
    <owner name>, <group name>, <size>, <date and time>,
    <pathname>

You can just use ls -l %t | cut -d ' ' -f 5 to retrieve the size field.

I'll try this, thanks!

[grimar]

In D65140#1600579, @grimar wrote:

In D65140#1600570, @MaskRay wrote:

If you just want to check the size of a file, just use ls -l. As POSIX says:

If the -l option is specified, the following information shall be written for files other than character special and block special files:

"%s %u %s %s %u %s %s\n", <file mode>, <number of links>,
    <owner name>, <group name>, <size>, <date and time>,
    <pathname>

You can just use ls -l %t | cut -d ' ' -f 5 to retrieve the size field.

I'll try this, thanks!

It doesn't work as is on windows :) The reason seems is an additional space printed after permissions.
I.e. what is 5th field on linux, it is 6th field on windows because of that:

D:\Work2\llvm>ls -l D:\Work2\llvm\llvm_build\test\tools\yaml2obj\Output\elf-custom-null-section.yaml.tmp4
-rw-rw-rw- 1 anon 0 377 2019-07-25 11:50 D:\Work2\llvm\llvm_build\test\tools\yaml2obj\Output\elf-custom-null-section.yaml.tmp4

The output above ^^^ has double space after "rw-rw-rw-"

D:\Work2\llvm>ls -l D:\Work2\llvm\llvm_build\test\tools\yaml2obj\Output\elf-custom-null-section.yaml.tmp4 | cut -d " " -f 1
-rw-rw-rw-

D:\Work2\llvm>ls -l D:\Work2\llvm\llvm_build\test\tools\yaml2obj\Output\elf-custom-null-section.yaml.tmp4 | cut -d " " -f 2

D:\Work2\llvm>ls -l D:\Work2\llvm\llvm_build\test\tools\yaml2obj\Output\elf-custom-null-section.yaml.tmp4 | cut -d " " -f 3
1

D:\Work2\llvm>ls -l D:\Work2\llvm\llvm_build\test\tools\yaml2obj\Output\elf-custom-null-section.yaml.tmp4 | cut -d " " -f 4
anon

D:\Work2\llvm>ls -l D:\Work2\llvm\llvm_build\test\tools\yaml2obj\Output\elf-custom-null-section.yaml.tmp4 | cut -d " " -f 5
0

D:\Work2\llvm>ls -l D:\Work2\llvm\llvm_build\test\tools\yaml2obj\Output\elf-custom-null-section.yaml.tmp4 | cut -d " " -f 6
377

I had to use ls -l %t3 | tr -s ' ' | cut -d ' ' -f 5 finally.

[grimar]

• Stop using stat.

[MaskRay]

It doesn't work as is on windows :) The reason seems is an additional space printed after permissions. I.e. what is 5th field on linux, it is 6th field on windows because of that:

I had to use ls -l %t3 | tr -s ' ' | cut -d ' ' -f 5 finally.

OK... That Windows ls utility is non-conforming...

[jhenderson]

FWIW, rather than messing about with cut and tr, it might have been easier to just use FileCheck directly, i.e. just do ls -l | FileCheck %s, and then construct an appropriate regex to skip the irrelevant bits. I don't think it's a big deal either way, but I would probably find it easier to read, personally.

[grimar]

In D65140#1600755, @jhenderson wrote:

FWIW, rather than messing about with cut and tr, it might have been easier to just use FileCheck directly, i.e. just do ls -l | FileCheck %s, and then construct an appropriate regex to skip the irrelevant bits. I don't think it's a big deal either way, but I would probably find it easier to read, personally.

I invented the following. But it does not look simpler to me actually...

# RUN: yaml2obj --docnum=4 %s -o %t4
# RUN: ls -l %t3 > %t.txt
# RUN: ls -l %t4 >> %t.txt
# RUN: FileCheck %s --input-file=%t.txt --check-prefix=SIZE

# SIZE: {{^[^ ]*[ ]+[^ ]*[ ][^ ]*[ ][^ ]*[ ]}}[[FILESIZE:[^ ]*]]
# SIZE: [[FILESIZE]]

[jhenderson]

In D65140#1601111, @grimar wrote:

In D65140#1600755, @jhenderson wrote:

FWIW, rather than messing about with cut and tr, it might have been easier to just use FileCheck directly, i.e. just do ls -l | FileCheck %s, and then construct an appropriate regex to skip the irrelevant bits. I don't think it's a big deal either way, but I would probably find it easier to read, personally.

I invented the following. But it does not look simpler to me actually...

# RUN: yaml2obj --docnum=4 %s -o %t4
# RUN: ls -l %t3 > %t.txt
# RUN: ls -l %t4 >> %t.txt
# RUN: FileCheck %s --input-file=%t.txt --check-prefix=SIZE

# SIZE: {{^[^ ]*[ ]+[^ ]*[ ][^ ]*[ ][^ ]*[ ]}}[[FILESIZE:[^ ]*]]
# SIZE: [[FILESIZE]]

I think you might be right, fair enough!

test/tools/yaml2obj/elf-custom-null-section.yaml

- Type:         SHT_NULL
     Flags:        [ ]
     AddressAlign: 0x0
     Size:         0x0
     EntSize:      0x0
     Link:         0
     Info:         0
     Address:      0x0
 
-## Check we are still able to describe other sections too.
+## Check we can redefine fields of the first SHT_NULL section.
 
 # RUN: yaml2obj --docnum=3 %s -o %t3
-# RUN: llvm-readelf --sections %t3 | FileCheck %s --check-prefix=OTHER-SECTION
+# RUN: llvm-readelf --sections %t3 | FileCheck %s --check-prefix=REDEFINE
 
-# OTHER-SECTION:      Section Headers:
-# OTHER-SECTION-NEXT:   [Nr] Name      Type     Address          Off    Size   ES Flg Lk Inf Al
-# OTHER-SECTION-NEXT:   [ 0]           NULL     0000000000000000 000000 000000 00 0   0  0
-# OTHER-SECTION-NEXT:   [ 1] foo       PROGBITS 0000000000000000 000180 000000 00 0   0  0
-# OTHER-SECTION-NEXT:   [ 2] .symtab   SYMTAB   0000000000000000 000180 000018 18 3   1  8
-# OTHER-SECTION-NEXT:   [ 3] .strtab   STRTAB   0000000000000000 000198 000001 00 0   0  1
-# OTHER-SECTION-NEXT:   [ 4] .shstrtab STRTAB   0000000000000000 000199 00001f 00 0   0  1
+# REDEFINE:      Section Headers:
+# REDEFINE-NEXT:  [Nr] Name Type Address          Off    Size   ES Flg Lk Inf Al
+# REDEFINE-NEXT:  [ 0] .foo NULL 0000000000000006 000000 000002 03   A 4   5  1
 
 --- !ELF
 FileHeader:
   Class:   ELFCLASS64
   Data:    ELFDATA2LSB
   Type:    ET_REL
   Machine: EM_X86_64
 Sections:
   - Type:         SHT_NULL
-    Name:         ''
-    Flags:        [ ]
-    AddressAlign: 0x0
-    Size:         0x0
-    EntSize:      0x0
-    Link:         0
-  - Type: SHT_PROGBITS
-    Name: 'foo'
+    Name:         .foo
+    Flags:        [ SHF_ALLOC ]
+    AddressAlign: 0x1
+    Size:         0x2
+    EntSize:      0x3
+    Link:         4
+    Info:         5
+    Address:      0x6
 
-## Check we can redefine sh_size and sh_link fields of the SHT_NULL section.
+## Check that file size does not change if we redefine the Size
+## of the first SHT_NULL section.
 
 # RUN: yaml2obj --docnum=4 %s -o %t4
-# RUN: llvm-readelf --sections %t4 | FileCheck %s --check-prefix=REDEFINE
+# RUN: ls -l %t3 | tr -s ' ' | cut -d ' ' -f 5 > %t.txt
+# RUN: ls -l %t4 | tr -s ' ' | cut -d ' ' -f 5 >> %t.txt
+# RUN: FileCheck %s --input-file=%t.txt --check-prefix=SIZE
 
-# REDEFINE:      Section Headers:
-# REDEFINE-NEXT:  [Nr] Name Type Address          Off    Size   ES Flg Lk Inf Al
-# REDEFINE-NEXT:  [ 0]      NULL 0000000000000000 000000 000123 00     1   0  0
+# SIZE: [[FILESIZE:.*]]
+# SIZE: [[FILESIZE]]
 
 --- !ELF
 FileHeader:
   Class:   ELFCLASS64
   Data:    ELFDATA2LSB
   Type:    ET_REL
   Machine: EM_X86_64
 Sections:
-  - Type: SHT_NULL
-    Link: .foo
-    Size: 0x123
-  - Type: SHT_PROGBITS
-    Name: .foo
+  - Type:         SHT_NULL
+    Name:         .foo
+    Flags:        [ SHF_ALLOC ]
+    AddressAlign: 0x1
+    Size:         0xFFFF
+    EntSize:      0x3
+    Link:         4
+    Info:         5
+    Address:      0x6
 
-## The same as above, but using a number as a Link value.
+## Check we are still able to describe other sections too.
 
 # RUN: yaml2obj --docnum=5 %s -o %t5
-# RUN: llvm-readelf --sections %t5 | FileCheck %s --check-prefix=REDEFINE
+# RUN: llvm-readelf --sections %t5 | FileCheck %s --check-prefix=OTHER-SECTION
+
+# OTHER-SECTION:      Section Headers:
+# OTHER-SECTION-NEXT:   [Nr] Name      Type     Address          Off    Size   ES Flg Lk Inf Al
+# OTHER-SECTION-NEXT:   [ 0]           NULL     0000000000000000 000000 000000 00 0   0  0
+# OTHER-SECTION-NEXT:   [ 1] foo       PROGBITS 0000000000000000 000180 000000 00 0   0  0
+# OTHER-SECTION-NEXT:   [ 2] .symtab   SYMTAB   0000000000000000 000180 000018 18 3   1  8
+# OTHER-SECTION-NEXT:   [ 3] .strtab   STRTAB   0000000000000000 000198 000001 00 0   0  1
+# OTHER-SECTION-NEXT:   [ 4] .shstrtab STRTAB   0000000000000000 000199 00001f 00 0   0  1
 
 --- !ELF
 FileHeader:
   Class:   ELFCLASS64
   Data:    ELFDATA2LSB
   Type:    ET_REL
   Machine: EM_X86_64
 Sections:
-  - Type: SHT_NULL
-    Link: 1
-    Size: 0x123
+  - Type:         SHT_NULL
+    Name:         ''
+    Flags:        [ ]
+    AddressAlign: 0x0
+    Size:         0x0
+    EntSize:      0x0
+    Link:         0
   - Type: SHT_PROGBITS
-    Name: .foo
+    Name: 'foo'
 
 ## Check we report an error if null section sh_link field refers to an unknown section.
 
 # RUN: not yaml2obj --docnum=6 %s -o %t6 2>&1 | FileCheck %s --check-prefix=CASE4
 
 # CASE4: error: Unknown section referenced: '.foo' at YAML section ''.
 
 --- !ELF
 FileHeader:
   Class:   ELFCLASS64
   Data:    ELFDATA2LSB
   Type:    ET_REL

 ## Check we do not crash if we have more than one SHT_NULL section.
 
 # RUN: yaml2obj --docnum=8 %s -o %t8
 # RUN: llvm-readelf --sections %t8 | FileCheck %s --check-prefix=MULTIPLE
 
 # MULTIPLE:      Section Headers:
 # MULTIPLE-NEXT:  [Nr] Name Type Address          Off    Size   ES Flg Lk Inf Al
 # MULTIPLE-NEXT:  [ 0]      NULL 0000000000000000 000000 000000 00 0   0  0
-# MULTIPLE-NEXT:  [ 1]      NULL 0000000000000123 000180 000020 10 A   1  2   0
+# MULTIPLE-NEXT:  [ 1] .foo NULL 0000000000000123 000180 000020 10 A   1  2   0
 
 --- !ELF
 FileHeader:
   Class:   ELFCLASS64
   Data:    ELFDATA2LSB
   Type:    ET_REL
   Machine: EM_X86_64
 Sections:
   - Type: SHT_NULL
   - Type:    SHT_NULL
+    Name:    .foo
     Flags:   [ SHF_ALLOC ]
     Size:    0x20
     EntSize: 0x10
     Link:    1
     Info:    2
     Address: 0x123
+
+## Check we can override the sh_offset/sh_size fields of the first SHT_NULL section if requested.
+
+# RUN: yaml2obj --docnum=9 %s -o %t9
+# RUN: llvm-readelf --sections %t9 | FileCheck %s --check-prefix=OVERRIDE
+
+# OVERRIDE:      Section Headers:
+# OVERRIDE-NEXT:  [Nr] Name Type Address          Off    Size   ES Flg Lk Inf Al
+# OVERRIDE-NEXT:  [ 0]      NULL 0000000000000000 000007 000008 00   0 0  0
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_X86_64
+Sections:
+  - Type:     SHT_NULL
+    Size:     0x2
+    ShOffset: 0x7
+    ShSize:   0x8

tools/yaml2obj/yaml2elf.cpp

void ELFState<ELFT>::initELFHeader(Elf_Ehdr &Header) {
   Header.e_shentsize =
       Doc.Header.SHEntSize ? (uint16_t)*Doc.Header.SHEntSize : sizeof(Elf_Shdr);
   // Immediately following the ELF header and program headers.
   Header.e_shoff =
       Doc.Header.SHOffset
           ? (typename ELFT::uint)(*Doc.Header.SHOffset)
           : sizeof(Header) + sizeof(Elf_Phdr) * Doc.ProgramHeaders.size();
   Header.e_shnum =
-      Doc.Header.SHNum ? (uint16_t)*Doc.Header.SHNum : SN2I.size() + 1;
+      Doc.Header.SHNum ? (uint16_t)*Doc.Header.SHNum : Doc.Sections.size();
   Header.e_shstrndx = Doc.Header.SHStrNdx ? (uint16_t)*Doc.Header.SHStrNdx
                                           : SN2I.get(".shstrtab");
 }
 
 template <class ELFT>
 void ELFState<ELFT>::initProgramHeaders(std::vector<Elf_Phdr> &PHeaders) {
   for (const auto &YamlPhdr : Doc.ProgramHeaders) {
     Elf_Phdr Phdr;

 bool ELFState<ELFT>::initSectionHeaders(ELFState<ELFT> &State,
                                         std::vector<Elf_Shdr> &SHeaders,
                                         ContiguousBlobAccumulator &CBA) {
   // Ensure SHN_UNDEF entry is present. An all-zero section header is a
   // valid SHN_UNDEF entry since SHT_NULL == 0.
   SHeaders.resize(Doc.Sections.size());
 
   for (size_t I = 0; I < Doc.Sections.size(); ++I) {
-    Elf_Shdr &SHeader = SHeaders[I];
     ELFYAML::Section *Sec = Doc.Sections[I].get();
-
-    if (I == 0) {
-      if (Sec->IsImplicit)
-        continue;
-
-      if (auto S = dyn_cast<ELFYAML::RawContentSection>(Sec))
-        if (S->Size)
-          SHeader.sh_size = *S->Size;
-
-      if (!Sec->Link.empty()) {
-        unsigned Index;
-        if (!convertSectionIndex(SN2I, Sec->Name, Sec->Link, Index))
-          return false;
-        SHeader.sh_link = Index;
-      }
+    if (I == 0 && Sec->IsImplicit)
       continue;
-    }
 
     // We have a few sections like string or symbol tables that are usually
     // added implicitly to the end. However, if they are explicitly specified
     // in the YAML, we need to write them here. This ensures the file offset
     // remains correct.
+    Elf_Shdr &SHeader = SHeaders[I];
     if (initImplicitHeader(State, CBA, SHeader, Sec->Name,
                            Sec->IsImplicit ? nullptr : Sec))
       continue;
 
     assert(Sec && "It can't be null unless it is an implicit section. But all "
                   "implicit sections should already have been handled above.");
 
     SHeader.sh_name = DotShStrtab.getOffset(dropUniqueSuffix(Sec->Name));
     SHeader.sh_type = Sec->Type;
     if (Sec->Flags)
       SHeader.sh_flags = *Sec->Flags;
     SHeader.sh_addr = Sec->Address;
     SHeader.sh_addralign = Sec->AddressAlign;
 
     if (!Sec->Link.empty()) {
       unsigned Index;
       if (!convertSectionIndex(SN2I, Sec->Name, Sec->Link, Index))
         return false;
       SHeader.sh_link = Index;
     }
 
-    if (auto S = dyn_cast<ELFYAML::RawContentSection>(Sec)) {
+    if (I == 0) {
+      if (auto RawSec = dyn_cast<ELFYAML::RawContentSection>(Sec)) {
+        // We do not write any content for special SHN_UNDEF section.
+        if (RawSec->Size)
+          SHeader.sh_size = *RawSec->Size;
+        if (RawSec->Info)
+          SHeader.sh_info = *RawSec->Info;
+      }
+      if (Sec->EntSize)
+        SHeader.sh_entsize = *Sec->EntSize;
+    } else if (auto S = dyn_cast<ELFYAML::RawContentSection>(Sec)) {
       if (!writeSectionContent(SHeader, *S, CBA))
         return false;
     } else if (auto S = dyn_cast<ELFYAML::RelocationSection>(Sec)) {
       if (!writeSectionContent(SHeader, *S, CBA))
         return false;
     } else if (auto S = dyn_cast<ELFYAML::Group>(Sec)) {
       if (!writeSectionContent(SHeader, *S, CBA))
         return false;
bool ELFState<ELFT>::writeSectionContent(Elf_Shdr &SHeader,
   }
   if (Section.Content)
     Section.Content->writeAsBinary(OS);
 
   return true;
 }
 
 template <class ELFT> bool ELFState<ELFT>::buildSectionIndex() {
-  for (unsigned I = 1, E = Doc.Sections.size(); I != E; ++I) {
+  for (unsigned I = 0, E = Doc.Sections.size(); I != E; ++I) {
     StringRef Name = Doc.Sections[I]->Name;
+    if (Name.empty())
+      continue;
+
     DotShStrtab.add(dropUniqueSuffix(Name));
     if (!SN2I.addName(Name, I)) {
       WithColor::error() << "Repeated section name: '" << Name
                          << "' at YAML section number " << I << ".\n";
       return false;
     }
   }