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runtime: Fix sigaltstack call with musl on certain Intel CPUs #21

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Merged
nmote merged 1 commit into from
Jul 14, 2026
Merged

runtime: Fix sigaltstack call with musl on certain Intel CPUs #21

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26 changes: 23 additions & 3 deletions runtime/signals.c
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Original file line number Diff line number Diff line change
Expand Up @@ -21,6 +21,9 @@
#include <errno.h>
#include <stdbool.h>
#include "caml/config.h"
#ifdef HAS_UNISTD
#include <unistd.h>
#endif
#ifdef USE_MMAP_MAP_STACK
#include <sys/mman.h>
#endif
Expand Down Expand Up @@ -522,12 +525,29 @@ CAMLexport int caml_rev_convert_signal_number(int signo)
return signo;
}

#ifdef POSIX_SIGNALS
/* musl (>= 1.2.6) rejects sigaltstack() when ss_size <
* sysconf(_SC_MINSIGSTKSZ). On some architectures, this can be larger than
* SIGSTKSZ which in musl is a build-time constant. */
static size_t caml_signal_stack_size(void)
{
size_t size = SIGSTKSZ;
#ifdef _SC_MINSIGSTKSZ
/* glibc/musl only */
long min = sysconf(_SC_MINSIGSTKSZ);

@dijkstracula dijkstracula Jul 14, 2026

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Do we want/need to handle sysconf returning -1 (and setting errno)? I think this would only happen if _SC_MINSIGSTKSZ is not a valid system limit, which you gate via the macro, so maybe not a concern but just throwing it out there.

@nmote nmote Jul 14, 2026

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Yes, and we already do (see the line below this one). I believe that it's possible for _SC_MINSIGSTKSZ to be a valid constant at build time but for the built binary to link against a different libc for which it is not a valid constant. In that case, sysconf would error, but this is handled by the min > 0 check below.

if (min > 0 && (size_t) min > size)
size = (size_t) min;
#endif
return size;
}
#endif /* POSIX_SIGNALS */

void * caml_init_signal_stack(void)
{
#ifdef POSIX_SIGNALS
stack_t stk;
stk.ss_flags = 0;
stk.ss_size = SIGSTKSZ;
stk.ss_size = caml_signal_stack_size();
/* The memory used for the alternate signal stack must not free'd before
calling sigaltstack with SS_DISABLE. malloc/mmap is therefore used rather
than caml_stat_alloc_noexc so that if a shutdown path erroneously fails
Expand All @@ -540,7 +560,7 @@ void * caml_init_signal_stack(void)
if (stk.ss_sp == MAP_FAILED)
return NULL;
if (sigaltstack(&stk, NULL) < 0) {
munmap(stk.ss_sp, SIGSTKSZ);
munmap(stk.ss_sp, stk.ss_size);
return NULL;
}
#else
Expand Down Expand Up @@ -577,7 +597,7 @@ void caml_free_signal_stack(void * signal_stack)
/* Memory was allocated with malloc/mmap directly (see
caml_init_signal_stack) */
#ifdef USE_MMAP_MAP_STACK
munmap(signal_stack, SIGSTKSZ);
munmap(signal_stack, caml_signal_stack_size());
#else
free(signal_stack);
#endif /* USE_MMAP_MAP_STACK */
Expand Down
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