sizeof (siginfo_t) problem

Jakub Jelinek (
Mon, 14 Jul 2003 08:40:00 -0400


When siginfo_t was added, the intent obviously was that its size
be 128 bytes on all arches.
This is true on all 32-bit arches and what glibc has in its siginfo_t
definition on all arches:
# if __WORDSIZE == 64
# define __SI_PAD_SIZE ((__SI_MAX_SIZE / sizeof (int)) - 4)
# else
# define __SI_PAD_SIZE ((__SI_MAX_SIZE / sizeof (int)) - 3)
# endif

typedef struct siginfo
int si_signo; /* Signal number. */
int si_errno; /* If non-zero, an errno value associated with
this signal, as defined in <errno.h>. */
int si_code; /* Signal code. */

int _pad[__SI_PAD_SIZE];
void *si_addr; /* Faulting insn/memory ref. */
} _sigfault;
} _sifields;
} siginfo_t;

The kernel unfortunately does this right on sparc64 and alpha from 64-bit
arches only; ia64, s390x, ppc64 etc. got it wrong.

This is a bad problem, since e.g. sigwaitinfo(3) is supported ABI,
and apps - as they are compiled against glibc headers, not kernel ones -
will thus reserve on the stack 8 bytes less than the kernel fills
(this is hidden in copy_siginfo_to_user, so usually only if si_code >= 0
all 136 bytes will be copied).
Note that glibc had such siginfo_t definitions from the beggining,
ie. it is not something changed recently. E.g. on s390x,
such definition was already in -r1.1 checkin in March 2001, x86_64/ia64
never had its own siginfo.h but used a generic one which had the
above __SI_PAD_SIZE definition since early 2000 (x86_64 was added in 2001,
ia64 about 5 months later than that change).

I have noticed this on s390x in a different place:
MD_FALLBACK_FRAME_STATE_FOR in gcc, in order to unwind through
signal frames, needs to locate ucontext, and looking at
stack pointer + 8 + sizeof(siginfo_t) (which works on s390 32-bit)
did not work at all, everything was shifted by 8 bytes.
This though means that the kernel siginfo_t change cannot be done
just in asm-*/siginfo.h headers - at least places where siginfo_t
is present within some structures ever visible to userland a dummy
8 byte pad needs to be inserted.

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