clang 17 fails to align stack struct with -fstack-protector-strong, building smartmontools

[citrus-it]

I searched for any similar issues to this but didn't find anything, so apologies if it's a known issue.

We recently switched the default clang compiler in the OmniOS operating system distribution from 16 to 17, and we had a report that the smartmontools package was no longer working correctly.

There is more detail in omniosorg/omnios-extra#1418 but the relevant parts are included below.

The short version is that clang 17 is not aligning drive on the stack for some reason in the following code if -fstack-protector-strong is in the build flags:

  ata_identify_device drive;
  memset(&drive, 0, sizeof(drive));
  unsigned char raw_drive[sizeof(drive)];
  memset(&raw_drive, 0, sizeof(raw_drive));

Here's the context.

When building with clang 17.0.6, the resulting disassembly shows the misalignment:

_Z12ataPrintMainP10ata_deviceRK17ata_print_options+0x3ef:       leaq   0xfffffffffffffdc6(%rbp),%rbx

which is supported by gdb's interpretation of the DWARF - note that raw_drive is aligned as expected.

Symbol drive is a variable at frame base reg $rbp offset 0+-570, length 512.
Symbol raw_drive is a variable at frame base reg $rbp offset 0+-1088, length 512.

Building with either clang 16 or 18 does not cause this, in both of those cases the variable is properly aligned, as shown below.

_Z12ataPrintMainP10ata_deviceRK17ata_print_options+0x3ef:       leaq   0xfffffffffffffdc0(%rbp),%rbx

Unfortunately I have not been able to come up with a standalone reproducer for this, but I did build the same smartmontools source on Ubuntu 24.04 with clang 17.0.6, and got similar results:

The build line for these was:

clang-17 -DHAVE_CONFIG_H -I.  -DBUILD_INFO='"(local build)"' -DSMARTMONTOOLS_SYSCONFDIR='"/usr/local/etc"' -DSMARTMONTOOLS_SMARTDSCRIPTDIR='"/usr/local/etc"' -DSMARTMONTOOLS_DRIVEDBDIR='"/usr/local/share/smartmontools"'      -D_FORTIFY_SOURCE=3  -g -O2 -std=gnu++11 -Wall -W -Wformat=2 -Werror=return-type -fstack-protector-strong -MT ataprint.o -MD -MP -MF .deps/ataprint.Tpo -c -o ataprint.o ataprint.cpp

clang17

   0x000000000000049c <+1068>:    lea    0x6ec7(%rsp),%rbx

clang18

   0x0000000000000496 <+1062>:    lea    0x6ef0(%rsp),%rbx

If there is any more data I can gather on this that would help, please let me know.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Andy Fiddaman (citrus-it)

I searched for any similar issues to this but didn't find anything, so apologies if it's a known issue.

We recently switched the default clang compiler in the OmniOS operating system distribution from 16 to 17, and we had a report that the smartmontools package was no longer working correctly.

There is more detail in omniosorg/omnios-extra#1418 but the relevant parts are included below.

The short version is that clang 17 is not aligning drive on the stack for some reason in the following code if -fstack-protector-strong is in the build flags:

  ata_identify_device drive;
  memset(&drive, 0, sizeof(drive));
  unsigned char raw_drive[sizeof(drive)];
  memset(&raw_drive, 0, sizeof(raw_drive));

Here's the context.

When building with clang 17.0.6, the resulting disassembly shows the misalignment:

_Z12ataPrintMainP10ata_deviceRK17ata_print_options+0x3ef:       leaq   0xfffffffffffffdc6(%rbp),%rbx

which is supported by gdb's interpretation of the DWARF - note that raw_drive is aligned as expected.

Symbol drive is a variable at frame base reg $rbp offset 0+-570, length 512.
Symbol raw_drive is a variable at frame base reg $rbp offset 0+-1088, length 512.

Building with either clang 16 or 18 does not cause this, in both of those cases the variable is properly aligned, as shown below.

_Z12ataPrintMainP10ata_deviceRK17ata_print_options+0x3ef:       leaq   0xfffffffffffffdc0(%rbp),%rbx

Unfortunately I have not been able to come up with a standalone reproducer for this, but I did build the same smartmontools source on Ubuntu 24.04 with clang 17.0.6, and got similar results:

The build line for these was:

clang-17 -DHAVE_CONFIG_H -I.  -DBUILD_INFO='"(local build)"' -DSMARTMONTOOLS_SYSCONFDIR='"/usr/local/etc"' -DSMARTMONTOOLS_SMARTDSCRIPTDIR='"/usr/local/etc"' -DSMARTMONTOOLS_DRIVEDBDIR='"/usr/local/share/smartmontools"'      -D_FORTIFY_SOURCE=3  -g -O2 -std=gnu++11 -Wall -W -Wformat=2 -Werror=return-type -fstack-protector-strong -MT ataprint.o -MD -MP -MF .deps/ataprint.Tpo -c -o ataprint.o ataprint.cpp

clang17

   0x000000000000049c <+1068>:    lea    0x6ec7(%rsp),%rbx

clang18

   0x0000000000000496 <+1062>:    lea    0x6ef0(%rsp),%rbx

If there is any more data I can gather on this that would help, please let me know.