| author |  Matthew Lugg <mlugg@mlugg.co.uk> 2026-01-06 11:05:50+00:00 |
| committer | Matthew Lugg <mlugg@mlugg.co.uk> 2026-01-06 11:15:33+00:00 |
| log | 073ef0f393ff4297e8d48a72f30a605c2d272289 |
| tree | fd179193b6bd11d0eb981f3add5923f9fca3a22f |
| parent | 111165513156d5732d85e5ccb52b9d8bded41ffa |
| signature |  Commit is signed but in an unrecognized format. |
std.Thread: mask all signals before unmapping stack
As the comment explains, if a signal were to arrive between a detached
thread's `munmap` and `exit` calls, the signal handler would immediately
trigger SIGSEGV due to the stack being unmapped. To solve this, we need
to block all signals before entering this logic. The musl implementation
which this logic was ported from does this exact thing; that logic was
just lost when porting.
Notably, this would lead to a crash with no stack trace, because the
SIGSEGV handler would itself crash due to the missing stack.1 files changed, 4 insertions(+), 0 deletions(-)
lib- std
lib/std/Thread.zig+4
| ... | @@ -1224,6 +1224,10 @@ const LinuxThreadImpl = struct { | ... | @@ -1224,6 +1224,10 @@ const LinuxThreadImpl = struct { |
| 1224 | /// Ported over from musl libc's pthread detached implementation: | 1224 | /// Ported over from musl libc's pthread detached implementation: |
| 1225 | /// https://github.com/ifduyue/musl/search?q=__unmapself | 1225 | /// https://github.com/ifduyue/musl/search?q=__unmapself |
| 1226 | fn freeAndExit(self: *ThreadCompletion) noreturn { | 1226 | fn freeAndExit(self: *ThreadCompletion) noreturn { |
| 1227 | // If a signal were delivered between SYS_munmap and SYS_exit, any installed signal | ||
| 1228 | // handler would immediately segfault due to the stack being unmapped. To avoid this, | ||
| 1229 | // we need to mask all signals before entering the inline asm. | ||
| 1230 | posix.sigprocmask(std.posix.SIG.BLOCK, &std.os.linux.sigfillset(), null); | ||
| 1227 | switch (target.cpu.arch) { | 1231 | switch (target.cpu.arch) { |
| 1228 | .x86 => asm volatile ( | 1232 | .x86 => asm volatile ( |
| 1229 | \\ movl $91, %%eax # SYS_munmap | 1233 | \\ movl $91, %%eax # SYS_munmap |