BlockImpTest failures with Clang 8 on RHEL8

[woachk]

On latest master:

`98% tests passed, 4 tests failed out of 178

Total Test time (real) = 29.15 sec

The following tests FAILED:
25 - BlockImpTest (SEGFAULT)
26 - BlockImpTest_optimised (SEGFAULT)
27 - BlockImpTest_legacy (SEGFAULT)
28 - BlockImpTest_legacy_optimised (SEGFAULT)`

`$ lldb Test/BlockImpTest
(lldb) target create "Test/BlockImpTest"
Current executable set to 'Test/BlockImpTest' (x86_64).
(lldb) run
Process 4784 launched: '/home/ares/labs2/libobjc2/Build/Test/BlockImpTest' (x86_64)
Process 4784 stopped

• thread 1.7.1 release? #1, name = 'BlockImpTest', stop reason = signal SIGSEGV: address access protected (fault address: 0x652000)
  frame #0: 0x0000000000652000
  -> 0x652000: movq -0x1007(%rip), %rsi
  0x652007: xchgq %rsi, %rdi
  0x65200a: jmpq *-0x1008(%rip)
  0x652010: movq -0x1007(%rip), %rsi
  (lldb) bt
• thread 1.7.1 release? #1, name = 'BlockImpTest', stop reason = signal SIGSEGV: address access protected (fault address: 0x652000)
  • frame #0: 0x0000000000652000
    frame 1.7.1 release? #1: 0x00000000004016fa BlockImpTestmain at BlockImpTest.m:38:2 frame #2: 0x00007ffff75d9873 libc.so.6.annobin_libc_start.c + 243
    frame Fix returning argument types from methods. #3: 0x000000000040140e BlockImpTest.annobin_init.c.hot + 46

[davidchisnall]

Do you have any unusual SELinux settings enabled? The block-to-imp code has been tested (in CI) on x86-64 Linux, but it may be that you have a setting that prevents changing permission of pages from read-write to read-execute.

[woachk]

Hello,

I use the default SELinux policy for CentOS 8. I can however confirm that all the tests pass with SELinux off. (tested it on another machine using the same OS)

For reference, Clang version is:
$ clang -v
clang version 8.0.1 (Red Hat 8.0.1-1.module_el8.1.0+215+a01033fb)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/8
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/8
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64

Thanks,

[ngrewe]

A quick check reveals that using mmap() with MAP_ANONYMOUS makes things work under CentOS 8's SELinux policy. As far as I'm aware, MAP_ANONYMOUS is not portable, though.

diff --git a/block_to_imp.c b/block_to_imp.c
index 655489a..8377623 100644
--- a/block_to_imp.c
+++ b/block_to_imp.c
@@ -163,7 +163,7 @@ static struct trampoline_set *alloc_trampolines(char *start, char *end)
 #if _WIN32
        metadata->buffers = VirtualAlloc(NULL, sizeof(struct trampoline_buffers), MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
 #else
-       posix_memalign((void **)&metadata->buffers, getpagesize(), sizeof(struct trampoline_buffers));
+       metadata->buffers = mmap(NULL, sizeof(struct trampoline_buffers), PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
 #endif
        for (int i=0 ; i<HEADERS_PER_PAGE ; i++)
        {

[davidchisnall]

Interesting. I'm not sure why we're using posix_memalign here. Please can you send a PR that replaces it with mmap? There's no reason why it can't use mmap, the Windows code path uses the Windows equivalent.