[Compiler-rt][MSan]Fix shmat testcase: Pass SHMLBA-alligned address to shmat

[mohit.bhakkad]

Summary

shmat() returns address which is aligned to SHMLBA, so while calling we need to pass SHMLBA-alligned address, to get equal addresses.

[mohit.bhakkad]

@eugenis could you please review this.

[samsonov]

Instead of writing different code for x86 and MIPS, can you just make sure that you get a SHMLBA-aligned address?

const int kShmSize = 4096;
void *mapping_start = mmap(NULL, kShmSize + SHMLBA, ...);
uptr p = (uptr)mapping_start;
if (p % SHMLBA != 0) {
  p = (p + SHMLBA - 1) / SHMLBA * SHMLBA;
}
// p is now SHMLBA-aligned;

[mohit.bhakkad]

Updated with suggested change, thanks!

[samsonov]

lib/msan/tests/msan_test.cc

1229

Better cast the right-hand side to void*

1229

s/4095/kShmSize - 1

1231

Hm? Shouldn't you unmap the original mmaped region of size kShmSize + SHMLBA?

1242

kShmSize - 1?

[mohit.bhakkad]

Updated with suggested changes.

[samsonov]

LGTM after fixing a bug below.

lib/msan/tests/msan_test.cc

1222

Shouldn't this be

void *p = mapping_start;

?

Also, you can just unconditionally round up:

void *p = (void*)(((unsigned long)mapping_start + SHMLBA - 1) / SHMLBA * SHMLBA);

[mohit.bhakkad]

lib/msan/tests/msan_test.cc

1222

Thanks for pointing this out, I will just keep this assignment unconditional.

lib/msan/tests/msan_test.cc

#if !defined(__FreeBSD__)
   EXPECT_NOT_POISONED(s_i);
 #endif
 
   res = shmctl(id, IPC_RMID, 0);
   ASSERT_GT(res, -1);
 }
 
 TEST(MemorySanitizer, shmat) {
-  void *p = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
+  const int kShmSize = 4096;
+  void *mapping_start = mmap(NULL, kShmSize + SHMLBA, PROT_READ | PROT_WRITE,
                              MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
-  ASSERT_NE(MAP_FAILED, p);
+  ASSERT_NE(MAP_FAILED, mapping_start);
+  
+  void *p;
+  if ((unsigned long)mapping_start % SHMLBA != 0) {
+    p = (void *)(((unsigned long)mapping_start + SHMLBA - 1) / SHMLBA * SHMLBA);
+  }
+  // p is now SHMLBA-aligned;
 
   ((char *)p)[10] = *GetPoisoned<U1>();
-  ((char *)p)[4095] = *GetPoisoned<U1>();
+  ((char *)p)[kShmSize - 1] = *GetPoisoned<U1>();
 
-  int res = munmap(p, 4096);
+  int res = munmap(mapping_start, kShmSize + SHMLBA);
   ASSERT_EQ(0, res);
 
-  int id = shmget(IPC_PRIVATE, 4096, 0644 | IPC_CREAT);
+  int id = shmget(IPC_PRIVATE, kShmSize, 0644 | IPC_CREAT);
   ASSERT_GT(id, -1);
 
   void *q = shmat(id, p, 0);
   ASSERT_EQ(p, q);
 
   EXPECT_NOT_POISONED(((char *)q)[0]);
   EXPECT_NOT_POISONED(((char *)q)[10]);
-  EXPECT_NOT_POISONED(((char *)q)[4095]);
+  EXPECT_NOT_POISONED(((char *)q)[kShmSize - 1]);
 
   res = shmdt(q);
   ASSERT_EQ(0, res);
 
   res = shmctl(id, IPC_RMID, 0);
   ASSERT_GT(res, -1);
 }