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std.posix.faccessat: fallback to older faccessat syscall if possible #24900

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std.posix.faccessat: fallback to older faccessat syscall if possible #24900

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e18cbf4
std.os: restore plain linux.faccessat syscall wrapper, only use .facc…
rohlem Aug 18, 2025
b433373
std.posix.faccessatZ: return error.UnsupportedFlags on android (secco…
rohlem Aug 18, 2025
6e3fdb6
std.posix.faccessatZ: return error.UnsupportedFlags on too old linux …
rohlem Aug 18, 2025
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8 changes: 6 additions & 2 deletions lib/std/os/linux.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -1238,11 +1238,15 @@ pub fn access(path: [*:0]const u8, mode: u32) usize {
if (@hasField(SYS, "access")) {
return syscall2(.access, @intFromPtr(path), mode);
} else {
return syscall4(.faccessat, @as(usize, @bitCast(@as(isize, AT.FDCWD))), @intFromPtr(path), mode, 0);
return faccessat(AT.FDCWD, path, mode);
}
}

pub fn faccessat(dirfd: i32, path: [*:0]const u8, mode: u32, flags: u32) usize {
pub fn faccessat(dirfd: i32, path: [*:0]const u8, mode: u32) usize {
return syscall3(.faccessat, @as(usize, @bitCast(@as(isize, dirfd))), @intFromPtr(path), mode);
}

pub fn faccessat2(dirfd: i32, path: [*:0]const u8, mode: u32, flags: u32) usize {
return syscall4(.faccessat2, @as(usize, @bitCast(@as(isize, dirfd))), @intFromPtr(path), mode, flags);
}

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21 changes: 20 additions & 1 deletion lib/std/posix.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -5054,7 +5054,22 @@ pub fn faccessatZ(dirfd: fd_t, path: [*:0]const u8, mode: u32, flags: u32) Acces
} else if (native_os == .wasi and !builtin.link_libc) {
return faccessat(dirfd, mem.sliceTo(path, 0), mode, flags);
}
switch (errno(system.faccessat(dirfd, path, mode, flags))) {

const flags_unsupported = switch (system) {
linux => (builtin.abi.isAndroid() //standard android programs run in a seccomp sandbox that seems to trigger signal 31 (SIGSYS) for faccessat2
or builtin.target.os.isAtLeast(.linux, .{ .major = 5, .minor = 8, .patch = 0 }) != true), //faccessat2 was introduced in Linux 5.8
else => false,
};
const need_flags = (flags != 0);
if (need_flags and flags_unsupported) return error.UnsupportedFlags;
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@rohlem rohlem Aug 18, 2025 •
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So I forgot to check whether error.UnsupportedFlags is in the error set, and it's not.
I took it from the FanotifyInitError error set, because the description seemed to pretty much match the situation here.
Should we add error.UnsupportedFlags to the AccessError error set, or is one of the other errors close enough to be used here?
(On android it's basically a low-level permissions issue, so maybe error.PermissionDenied, but I kind of don't think users care about that technicality.)

const faccessat_result = switch (system) {
linux => if (need_flags) //the older, simpler syscall should stay supported, so we only need to use faccessat2 if we need flags
linux.faccessat2(dirfd, path, mode, flags)
else
linux.faccessat(dirfd, path, mode),
else => system.faccessat(dirfd, path, mode, flags),
};
switch (errno(faccessat_result)) {
.SUCCESS => return,
.ACCES => return error.AccessDenied,
.PERM => return error.PermissionDenied,
Expand All @@ -5072,6 +5087,10 @@ pub fn faccessatZ(dirfd: fd_t, path: [*:0]const u8, mode: u32, flags: u32) Acces
return error.InvalidUtf8
else
return unexpectedErrno(err),
.NOSYS => |err| if (system == linux)
return error.UnsupportedFlags
Comment on lines +5090 to +5091
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Under what conditions are we expecting to hit this branch?

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@rohlem rohlem Aug 25, 2025 •
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Building for a non-android target range including no support (expected before Linux kernel 5.8) and support (expected Linux kernel 5.8 and later),
being run on a Linux kernel without support (returning ENOSYS from the syscall).

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@rohlem rohlem Aug 25, 2025 •
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I looked at fchmodat2 in the same file for reference, which also does no separate kernel version check at runtime, and instead checks if the syscall fails with .NOSYS.
Two differences I only noticed now: fchmodat2 uses E.init(...) instead of errno(...), and it caches the support in a static (container-scoped) var (atomically accessed) to avoid repeated unsupported calls.

else
return unexpectedErrno(err),
else => |err| return unexpectedErrno(err),
}
}
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