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os: Add busfault_set_jmp() for SIGBUS unwind

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Allow callers to arm a sigjmp_buf on a registered mapping so the
SIGBUS handler can siglongjmp instead of remapping and retrying the
faulting instruction.

Needed when the mapping contains objects that are unsafe to resume
on after a replace.

This is preparation work for the next commit where that mechanism is
used.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Assisted-by: AI
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2270>
(cherry picked from commit 7a0b3e2197)
This commit is contained in:
Olivier Fourdan 2026-08-06 11:26:48 +02:00 • committed by Enrico Weigelt
commit e4b68f9248
2 changed files with 28 additions and 0 deletions
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18
os/busfault.c
View file

@ -35,6 +35,7 @@
#include "os/busfault.h"
#include <list.h>
#include <setjmp.h>
struct busfault {
struct xorg_list list;
@ -46,6 +47,8 @@ struct busfault {
busfault_notify_ptr notify;
void *context;
sigjmp_buf *jmp;
};
static Bool busfaulted;
@ -65,6 +68,7 @@ busfault_register_mmap(void *addr, size_t size, busfault_notify_ptr notify, void
busfault->notify = notify;
busfault->context = context;
busfault->valid = TRUE;
busfault->jmp = NULL;
xorg_list_add(&busfault->list, &busfaults);
return busfault;
@ -77,6 +81,13 @@ busfault_unregister(struct busfault *busfault)
free(busfault);
}
void
busfault_set_jmp(struct busfault *busfault, sigjmp_buf *jmp)
{
if (busfault)
busfault->jmp = jmp;
}
void
busfault_check(void)
{
@ -101,6 +112,7 @@ busfault_sigaction(int sig, siginfo_t *info, void *param)
void *fault = info->si_addr;
struct busfault *iter, *busfault = NULL;
void *new_addr;
sigjmp_buf *jmp;
/* Locate the faulting address in our list of shared segments
*/
@ -119,6 +131,12 @@ busfault_sigaction(int sig, siginfo_t *info, void *param)
busfault->valid = FALSE;
busfaulted = TRUE;
jmp = busfault->jmp;
if (jmp) {
busfault->jmp = NULL;
siglongjmp(*jmp, 1);
}
/* The client truncated the file; unmap the shared file, map
* /dev/zero over that area and keep going
*/

10
os/busfault.h
View file

@ -30,6 +30,7 @@ typedef void (*busfault_notify_ptr) (void *context);
#ifdef HAVE_SIGACTION
#include <sys/types.h>
#include <setjmp.h>
struct busfault *
busfault_register_mmap(void *addr, size_t size, busfault_notify_ptr notify, void *context);
@ -37,6 +38,15 @@ busfault_register_mmap(void *addr, size_t size, busfault_notify_ptr notify, void
void
busfault_unregister(struct busfault *busfault);
/*
* If jmp is non-NULL when a SIGBUS hits this mapping, the handler
* siglongjmps there instead of remapping and retrying the faulting
* instruction. Callers must use sigsetjmp(..., 1).
* Pass NULL to clear.
*/
void
busfault_set_jmp(struct busfault *busfault, sigjmp_buf *jmp);
void
busfault_check(void);