Re: [PATCH v1] gdb: align siginfo_t with the Linux kernel definition

[Matthieu Longo <matthieu.longo@arm.com>]

On 19/06/2026 11:45, Matthieu Longo wrote:
> On 19/06/2026 04:58, Thiago Jung Bauermann wrote:
>> Hello Matthieu,
>>
>> Matthieu Longo <matthieu.longo@arm.com> writes:
>>
>>> GDB's current definition of siginfo_t is missing many fields present in
>>> the Linux kernel definition [1].
>>>
>>> These fields are useful for providing detailed, user-friendly diagnostics
>>> when a fault occurs. Some new AArch64 extensions, such as Permission
>>> Overlay Enhancement used to implement Protection Keys [2], require the
>>> debugger to inspect 'si_pkey' alongside 'si_addr' to help the user identify
>>> the problematic key.
>>>
>>> This patch aligns GDB's definition of the __sifields._sigfault member of
>>> siginfo_t with the definition from the Linux kernel master branch.
>>>
>>> [1]: https://github.com/torvalds/linux/blob/2b414a95b8f7307d42173ba9e580d6
>>>       d3e2bcbfce/include/uapi/asm-generic/siginfo.h#L69-L100
>>
>> Nit: https://github.com/torvalds/linux/ is a mirror. It's better to use
>> the official address at
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>>
> 
> Fixed.
> 
>>> [2]: https://lore.kernel.org/all/20160212210213.ABC488FA@viggo.jf.intel.com/
>>> ---
>>>   gdb/linux-tdep.c | 39 +++++++++++++++++++++++++++++++++++++--
>>>   1 file changed, 37 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
>>> index a7381677498..5baad41a083 100644
>>> --- a/gdb/linux-tdep.c
>>> +++ b/gdb/linux-tdep.c
>>> @@ -275,7 +275,7 @@ linux_get_siginfo_type (struct gdbarch *gdbarch)
>>>     struct type *void_ptr_type;
>>>     struct type *uid_type, *pid_type;
>>>     struct type *sigval_type, *clock_type;
>>> -  struct type *siginfo_type, *sifields_type;
>>> +  struct type *siginfo_type, *sifields_type, *sigfault_union_type;
>>>     struct type *type;
>>>     linux_gdbarch_data = get_linux_gdbarch_data (gdbarch);
>>> @@ -285,9 +285,12 @@ linux_get_siginfo_type (struct gdbarch *gdbarch)
>>>     type_allocator alloc (gdbarch);
>>>     const struct builtin_type *builtin_types = builtin_type (gdbarch);
>>> +  struct type *short_type = builtin_types->builtin_short;
>>>     struct type *int_type = builtin_types->builtin_int;
>>>     struct type *uint_type = builtin_types->builtin_unsigned_int;
>>>     struct type *long_type = builtin_types->builtin_long;
>>> +  struct type *unsigned_long_type = builtin_types->builtin_unsigned_long;
>>> +  struct type *uint32_type = builtin_types->builtin_uint32;
>>>     void_ptr_type = lookup_pointer_type (builtin_type (gdbarch)->builtin_void);
>>> @@ -364,9 +367,41 @@ linux_get_siginfo_type (struct gdbarch *gdbarch)
>>>     append_composite_type_field (type, "si_stime", clock_type);
>>>     append_composite_type_field (sifields_type, "_sigchld", type);
>>> -  /* _sigfault */
>>> +  /* Begin _sigfault's anonymous union.  */
>>> +  sigfault_union_type = arch_composite_type (gdbarch, NULL, TYPE_CODE_UNION);
>>> +  /* used on alpha and sparc */
>>> +  append_composite_type_field (sigfault_union_type, "si_trapno", int_type);
>>> +  /* used when si_code=BUS_MCEERR_AR or used when si_code=BUS_MCEERR_AO */
>>> +  append_composite_type_field (sigfault_union_type, "si_addr_lsb", short_type);
>>> +
>>> +  /* used when si_code=SEGV_BNDERR */
>>> +  type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
>>> +  append_composite_type_field (type, "_dummy_bnd", short_type);
>>
>> The type of _dummy_bnd isn't that important in practice because the
>> field is just for padding, but its size is wrong AFAICS:
>>
>> $ cat siginfo_t.c
>> #include <stdio.h>
>> #include <asm/siginfo.h>
>>
>> int main (int argc, char *argv[]) {
>>      siginfo_t siginfo;
>>
>>      printf ("sizeof dummy_bnd = %lu\n",
>>              sizeof(siginfo._sifields._sigfault._addr_bnd._dummy_bnd));
>>
>>      return 0;
>> }
>> $ make siginfo_t
>> cc     siginfo_t.c   -o siginfo_t
>> $ ./siginfo_t
>> sizeof dummy_bnd = 8
>>
> 
> Sorry, I misread the macro __ADDR_BND_PKEY_PAD.
> 
>>> +  append_composite_type_field (type, "si_lower", void_ptr_type);
>>> +  append_composite_type_field (type, "si_upper", void_ptr_type);
>>> +  append_composite_type_field (sigfault_union_type, "_addr_bnd", type);
>>> +
>>> +  /* used when si_code=SEGV_PKUERR */
>>> +  type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
>>> +  append_composite_type_field (type, "_dummy_pkey", short_type);
>>
>> This one also has the wrong size, because <asm/siginfo.h> uses the same
>> __ADDR_BND_PKEY_PAD constant for the size of _dummy_bnd and of
>> _dummy_pkey.
>>
> 
> Fixed as follows:
> 
> diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
> index 5baad41a083..19289cf4b3f 100644
> --- a/gdb/linux-tdep.c
> +++ b/gdb/linux-tdep.c
> @@ -293,6 +293,13 @@ linux_get_siginfo_type (struct gdbarch *gdbarch)
>     struct type *uint32_type = builtin_types->builtin_uint32;
> 
>     void_ptr_type = lookup_pointer_type (builtin_type (gdbarch)->builtin_void);
> +  /* Compute padding length, i.e. __ADDR_BND_PKEY_PAD.  */
> +  unsigned alignof_void_ptr = type_align (void_ptr_type);
> +  unsigned padding_size = (alignof_void_ptr < sizeof(short)
> +                          ? sizeof(short)
> +                          : alignof_void_ptr);
> +  struct type *addr_bnd_pkey_padding_type
> +    = init_vector_type (builtin_types->builtin_uint8, padding_size);
> 
>     /* sival_t */
>     sigval_type = arch_composite_type (gdbarch, NULL, TYPE_CODE_UNION);
> @@ -376,14 +383,14 @@ linux_get_siginfo_type (struct gdbarch *gdbarch)
> 
>     /* used when si_code=SEGV_BNDERR */
>     type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
> -  append_composite_type_field (type, "_dummy_bnd", short_type);
> +  append_composite_type_field (type, "_dummy_bnd", addr_bnd_pkey_padding_type);
>     append_composite_type_field (type, "si_lower", void_ptr_type);
>     append_composite_type_field (type, "si_upper", void_ptr_type);
>     append_composite_type_field (sigfault_union_type, "_addr_bnd", type);
> 
>     /* used when si_code=SEGV_PKUERR */
>     type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
> -  append_composite_type_field (type, "_dummy_pkey", short_type);
> +  append_composite_type_field (type, "_dummy_pkey", addr_bnd_pkey_padding_type);
>     append_composite_type_field (type, "si_pkey", uint32_type);
>     append_composite_type_field (sigfault_union_type, "_addr_pkey", type);
> 
> And as expected on AArch64, the padding size is 8 bytes.
> 
> (gdb) print $_siginfo._sifields._sigfault
> $2 = {
>    si_addr = 0xfffff7ff5000,
>    _ = {
>      si_trapno = 0,
>      si_addr_lsb = 0,
>      _addr_bnd = {
>        _dummy_bnd = {0, 0, 0, 0, 0, 0, 0, 0},
>        si_lower = 0x3,
>        si_upper = 0x0
>      },
>      _addr_pkey = {
>        _dummy_pkey = {0, 0, 0, 0, 0, 0, 0, 0},
>        si_pkey = 3
>      },
>      _perf = {
>        si_perf_data = 0,
>        si_perf_type = 3,
>        si_perf_flags = 0
>      },
>      _tchange = {
>        si_tchange_target = 0,
>        si_tchange_type = 0,
>        si_tchange_flags = 3
>      }
>    }
> }
> (gdb) ptype $_siginfo._sifields._sigfault._._addr_pkey._dummy_pkey
> type = uint8_t __attribute__ ((vector_size(8)))
> 
> 
> Matthieu

By the way, I forgot to ask.
Would you prefer me to publish a v2 instead of the diff above ?

Matthieu