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std: fix os.linux.x86.syscall6

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It was possible for `arg6` to be passed as an operand relative to esp.
In that case, the `push` at the top clobbered esp and hence made the
reference to arg6 invalid. This was manifesting in this branch as broken
stack traces on x86-linux due to an `mmap2` syscall accidentally passing
the page offset as non-zero!

This commit fixes a bug introduced in cb0e6d8aa.
This commit is contained in:
mlugg 2025-09-11 16:50:48 +01:00
parent 9901b9389e
commit a12ce28224
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1 changed files with 15 additions and 11 deletions
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26
lib/std/os/linux/x86.zig
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@ -80,28 +80,32 @@ pub fn syscall6(
arg5: usize, arg5: usize,
arg6: usize, arg6: usize,
) usize { ) usize {
// arg5/arg6 are passed via memory as we're out of registers if ebp is used as frame pointer, or // arg6 can't be passed to asm in a register because ebp might be reserved as the frame pointer
// if we're compiling with PIC. We push arg5/arg6 on the stack before changing ebp/esp as the // and there are no more GPRs available; so we'll need a memory operand for it. Adding that
// compiler may reference arg5/arg6 as an offset relative to ebp/esp. // memory operand means that on PIC we might need a reference to the GOT, which in turn needs
// *its* own GPR, so we need to pass another arg in memory too! This is surprisingly hard to get
// right, because we can't touch esp or ebp until we're done with the memory input (as that
// input could be relative to esp or ebp).
const args56: [2]usize = .{ arg5, arg6 };
return asm volatile ( return asm volatile (
\\ push %[arg5] \\ push %[args56]
\\ push %[arg6]
\\ push %%edi
\\ push %%ebp \\ push %%ebp
\\ mov 12(%%esp), %%edi \\ mov 4(%%esp), %%ebp
\\ mov 8(%%esp), %%ebp \\ mov %%edi, 4(%%esp)
\\ // The saved %edi and %ebp are on the stack, and %ebp points to `args56`.
\\ // Prepare the last two args, syscall, then pop the saved %ebp and %edi.
\\ mov (%%ebp), %%edi
\\ mov 4(%%ebp), %%ebp
\\ int $0x80 \\ int $0x80
\\ pop %%ebp \\ pop %%ebp
\\ pop %%edi \\ pop %%edi
\\ add $8, %%esp
: [ret] "={eax}" (-> usize), : [ret] "={eax}" (-> usize),
: [number] "{eax}" (@intFromEnum(number)), : [number] "{eax}" (@intFromEnum(number)),
[arg1] "{ebx}" (arg1), [arg1] "{ebx}" (arg1),
[arg2] "{ecx}" (arg2), [arg2] "{ecx}" (arg2),
[arg3] "{edx}" (arg3), [arg3] "{edx}" (arg3),
[arg4] "{esi}" (arg4), [arg4] "{esi}" (arg4),
[arg5] "rm" (arg5), [args56] "rm" (&args56),
[arg6] "rm" (arg6),
: .{ .memory = true }); : .{ .memory = true });
} }