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

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

On 12/06/2026 11:16, Matthieu Longo wrote:
> 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
> [2]: https://lore.kernel.org/all/20160212210213.ABC488FA@viggo.jf.intel.com/
> ---
>   gdb/linux-tdep.c | 39 +++++++++++++++++++++++++++++++++++++--
>   1 file changed, 37 insertions(+), 2 deletions(-)
> 
I was not sure who I should include for the review. Please feel free to CC the right persons if they 
are not already in the list.

I tested the patch above, and it seems to work fine without any change in ./gdb/nat
However, in my understanding, compat_siginfo_t should also be impacted. Why is it still working then 
without any change ?

(gdb) print $_siginfo
$2 = {
   si_signo = 4,
   si_errno = 0,
   si_code = 12,
   _sifields = {
     _pad = {4327056, 0, 0, 1, 0 <repeats 24 times>},
     _kill = {
       si_pid = 4327056,
       si_uid = 0
     },
     _timer = {
       si_tid = 4327056,
       si_overrun = 0,
       si_sigval = {
         sival_int = 0,
         sival_ptr = 0x100000000
       }
     },
     _rt = {
       si_pid = 4327056,
       si_uid = 0,
       si_sigval = {
         sival_int = 0,
         sival_ptr = 0x100000000
       }
     },
     _sigchld = {
       si_pid = 4327056,
       si_uid = 0,
       si_status = 0,
       si_utime = 1,
       si_stime = 0
     },
     _sigfault = {
       si_addr = 0x420690 <tchangef(int)+16>,
       _ = {
         si_trapno = 0,
         si_addr_lsb = 0,
         _addr_bnd = {
           _dummy_bnd = 0,
           si_lower = 0x10000,
           si_upper = 0x0
         },
         _addr_pkey = {
           _dummy_pkey = 0,
           si_pkey = 65536
         },
         _perf = {
           si_perf_data = 4294967296,
           si_perf_type = 0,
           si_perf_flags = 0
         },
         _tchange = {
           si_tchange_target = 0,
           si_tchange_type = 1,
           si_tchange_flags = 0
         }
--Type <RET> for more, q to quit, c to continue without paging--

Note: this dump contains more fields than the Linux kernel master because it has some unmerged patch 
series on top of it. Please don't pay attention to it. That's just for the example.

aarch64_siginfo_from_compat_siginfo (in gdb/nat/aarch64-linux.c) and its friends for each backend 
seem to provide some conversion logic between the siginfo_t from the system 
(/usr/include/asm-generic/siginfo.h) and some internal representation compat_siginfo_t.
 From my research, this seems needed because, if the host and target systems are not using the same 
ABI (endianness, size of a pointer or int, etc...), GDB cannot rely on the local definition of 
siginfo_t to interpret correctly the data in the blob that it received from the target.

However, I am confused by the definition of compat_siginfo_t in gdb/nat/aarch64-linux.h for 
instance. I see that addresses like _sigfaul._addr are converted to 'unsigned int' (that should be 
equivalent to uint32_t I guess). Why is this working ?

Please, could you provide explanations on the above, and guidances on the changes required in struct 
compat_siginfo_t ?

Matthieu