[lldb] Refactor thread-step-out-ret-addr-check test to use .data instead of stack variable

[mossberg]

Summary

Original diff: D71372
Related diff: D71784

D71784 shows that we cannot assume the stack is not executable across platforms. Refactor the test to use a variable from the .data section which should be guaranteed non-executable.

I would appreciate guidance regarding the var@GOTPCREL(%rip). Directly referencing var would not assemble on my darwin machine due to an error about absolute addressing, and I used this to workaround it. I'm not sure if it's even portable to other platforms?

@jankratochvil Would you be able to test this on a Fedora machine, please?

[labath]

I don't really understand what's going on, but it seems that the lack of the .note.GNU-stack section causes the data section to be executable too (at least on my machine, anyway). Which means that using the data address will not help. :( I'm cc'ing some people who know more about dynamic linkers to see if anyone can explain this behavior...

One sure way to get a non-executable memory address would be to allocate something on the heap (in the C file), but I think @jankratochvil's approach of adding the .note.GNU-stack is simpler, and so far it seems that it works everywhere. The section directive will trip up the non-linux assemblers though, so it needs to be #ifdef __linux__ (afaict #ifdefs work just fine in asm files when invoking the processing through clang)...

lldb/test/Shell/Unwind/Inputs/thread-step-out-ret-addr-check.s

18

This should be 0x08, or you'll should push one more value to stack in asm_main.

[jankratochvil]

My testing matches the @labath's one. I have updated D71784.

[mossberg]

Thanks all for testing this! I had no idea it was this hard to get a non-executable section :)

Superseded by D71784

lldb/test/Shell/Unwind/Inputs/thread-step-out-ret-addr-check.s

         .text
         .globl  asm_main
 asm_main:
-        sub $0x8, %rsp
-        movq $0, (%rsp)
-        push %rsp
+        mov var@GOTPCREL(%rip), %rcx
+        push %rcx
         jmp _nonstandard_stub
 
 # Takes a single pointer argument via the stack, which is nonstandard for x64.
 # Executing 'thread step-out' here will initially attempt to write a
-# breakpoint to that stack address, but should fail because of the executable
+# breakpoint to that .data address, but should fail because of the executable
 # memory check.
 _nonstandard_stub:
         mov (%rsp), %rdi
         mov (%rdi), %rsi
         add $1, %rsi
         mov %rsi, (%rdi)
 
         add $0x10, %rsp
         ret
+
+        .data
+var:
+        .long 0