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

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

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