[libc] Swap order of syscall on chmod

[mikhailramalho]

We define SYS_fchmodat2 on libc but the syscall is not available on old kernels, so prefer the SYS_fchmodat version when possible.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Mikhail R. Gadelha (mikhailramalho)

Changes

We define SYS_fchmodat2 on libc but the syscall is not available on old kernels, so prefer the SYS_fchmodat version when possible.

---

Full diff: https://github.com/llvm/llvm-project/pull/138427.diff

1 Files Affected:

• (modified) libc/src/sys/stat/linux/chmod.cpp (+3-3)

diff --git a/libc/src/sys/stat/linux/chmod.cpp b/libc/src/sys/stat/linux/chmod.cpp
index 57d5bae6b8191..1b787e47e7c68 100644
--- a/libc/src/sys/stat/linux/chmod.cpp
+++ b/libc/src/sys/stat/linux/chmod.cpp
@@ -23,12 +23,12 @@ namespace LIBC_NAMESPACE_DECL {
 LLVM_LIBC_FUNCTION(int, chmod, (const char *path, mode_t mode)) {
 #ifdef SYS_chmod
   int ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_chmod, path, mode);
-#elif defined(SYS_fchmodat2)
-  int ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_fchmodat2, AT_FDCWD, path,
-                                              mode, 0, AT_SYMLINK_NOFOLLOW);
 #elif defined(SYS_fchmodat)
   int ret =
       LIBC_NAMESPACE::syscall_impl<int>(SYS_fchmodat, AT_FDCWD, path, mode, 0);
+#elif defined(SYS_fchmodat2)
+  int ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_fchmodat2, AT_FDCWD, path,
+                                              mode, 0, AT_SYMLINK_NOFOLLOW);
 #else
 #error "chmod, fchmodat and fchmodat2 syscalls not available."
 #endif

[michaelrj-google]

This patch is fine with me, but I'm a bit confused by the reasoning. If the fchmodat2 syscall isn't available from the kernel, shouldn't it also not be defined by the headers?

[mikhailramalho]

Yeah, maybe it's a weird kernel shipped for the bpi-f3?

mgadelha@archlinux include $ uname -a
Linux archlinux 6.1.15-legacy-k1 #9 SMP PREEMPT Mon Aug 12 15:06:24 UTC 2024 riscv64 GNU/Linux
mgadelha@archlinux include $ pwd
/usr/include
mgadelha@archlinux include $ grep -r fchmodat2 *
asm/unistd_32.h:#define __NR_fchmodat2 452
asm/unistd_64.h:#define __NR_fchmodat2 452
asm-generic/unistd.h:#define __NR_fchmodat2 452
asm-generic/unistd.h:__SYSCALL(__NR_fchmodat2, sys_fchmodat2)
bits/syscall.h:#ifdef __NR_fchmodat2
bits/syscall.h:# define SYS_fchmodat2 __NR_fchmodat2
seccomp-syscalls.h:#define __SNR_fchmodat2			__NR_fchmodat2
valgrind/vki/vki-scnums-mips32-linux.h:#define __NR_fchmodat2                  (__NR_Linux + 452)
valgrind/vki/vki-scnums-shared-linux.h:#define __NR_fchmodat2		452

The defines are there, and we crash when trying to do the syscall.