Re: [PATCH v5] gdb: align siginfo_t with the Linux kernel definition

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

On 12/08/2026 20:08, Simon Marchi wrote:
> On 7/28/26 8:32 AM, 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.
>>
>> To avoid hardcoding the field access paths throughout the codebase, this
>> patch also introduces compile-time accessors for the siginfo_t attributes,
>> centralizing their definitions in a single location and making future
>> updates easier.
>>
>> Finally, extend the testsuite to verify access to the new si_pkey field
>> and its preservation when modifying $_siginfo and when reading core files.
>> The tests in siginfo-obj.exp rely on the siginfo_t definition provided by
>> glibc's <signal.h>, which does not yet expose all of the fields present in
>> the kernel definition. As a result, the tests cannot exercise every newly
>> added field and therefore focus on si_pkey, the field motivating this change.
>> The test validates that GDB can read and modify the field correctly; it does
>> not attempt to generate a real protection-key fault.
>>
>> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
>>      tree/include/uapi/asm-generic/siginfo.h#n69
>> [2]: https://lore.kernel.org/all/20160212210213.ABC488FA@viggo.jf.intel.com/
>>
>> Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
>> ---
>>  gdb/aarch64-linux-tdep.c               |  8 ++--
>>  gdb/linux-tdep.c                       | 52 +++++++++++++++++++---
>>  gdb/linux-tdep.h                       | 60 ++++++++++++++++++++++++++
>>  gdb/sparc64-linux-tdep.c               |  6 ++-
>>  gdb/testsuite/gdb.base/siginfo-obj.c   |  1 +
>>  gdb/testsuite/gdb.base/siginfo-obj.exp | 14 ++++++
>>  6 files changed, 131 insertions(+), 10 deletions(-)
>>
>> diff --git a/gdb/aarch64-linux-tdep.c b/gdb/aarch64-linux-tdep.c
>> index f11eccc1bc1..235b35bcfb4 100644
>> --- a/gdb/aarch64-linux-tdep.c
>> +++ b/gdb/aarch64-linux-tdep.c
>> @@ -2683,13 +2683,15 @@ aarch64_linux_report_signal_info (struct gdbarch *gdbarch,
>>  
>>    try
>>      {
>> +      using gdb_si = gdb::siginfo_type;
>> +      using si_key = gdb::siginfo_type::key;
>>        /* Sigcode tells us if the segfault is actually a memory tag
>>  	 violation.  */
>> -      si_code = parse_and_eval_long ("$_siginfo.si_code");
>> -      si_errno = parse_and_eval_long ("$_siginfo.si_errno");
>> +      si_code = parse_and_eval_long (gdb_si::get (si_key::siginfo_code));
>> +      si_errno = parse_and_eval_long (gdb_si::get (si_key::siginfo_errno));
>>  
>>        fault_addr
>> -	= parse_and_eval_long ("$_siginfo._sifields._sigfault.si_addr");
>> +	= parse_and_eval_long (gdb_si::get (si_key::siginfo_addr));
>>      }
>>    catch (const gdb_exception_error &exception)
>>      {
>> diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
>> index 25d625db595..740043a9292 100644
>> --- a/gdb/linux-tdep.c
>> +++ b/gdb/linux-tdep.c
>> @@ -272,10 +272,9 @@ static struct type *
>>  linux_get_siginfo_type (struct gdbarch *gdbarch)
>>  {
>>    struct linux_gdbarch_data *linux_gdbarch_data;
>> -  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);
>> @@ -285,11 +284,22 @@ 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;
>> -
>> -  void_ptr_type = lookup_pointer_type (builtin_type (gdbarch)->builtin_void);
>> +  struct type *unsigned_long_type = builtin_types->builtin_unsigned_long;
>> +  struct type *uint32_type = builtin_types->builtin_uint32;
>> +  struct type *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 < short_type->length ()
>> +			   ? short_type->length ()
>> +			   : 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);
>> @@ -364,9 +374,41 @@ 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 is BUS_MCEERR_AR or 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", 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", addr_bnd_pkey_padding_type);
>> +  append_composite_type_field (type, "si_pkey", uint32_type);
>> +  append_composite_type_field (sigfault_union_type, "_addr_pkey", type);
>> +
>> +  /* used when si_code=TRAP_PERF */
>> +  type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
>> +  append_composite_type_field (type, "si_perf_data", unsigned_long_type);
>> +  append_composite_type_field (type, "si_perf_type", uint32_type);
>> +  append_composite_type_field (type, "si_perf_flags", uint32_type);
>> +  append_composite_type_field (sigfault_union_type, "_perf", type);
>> +
>> +  /* End _sigfault's anonymous union.  */
>> +
>> +  /* _sigfault is set by SIGILL, SIGFPE, SIGSEGV, SIGBUS, SIGTRAP, SIGEMT */
>>    type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
>>    append_composite_type_field (type, "si_addr", void_ptr_type);
>> +  /* Since there is no possibility to declare an anonymous union,
>> +     using '_anon_union' instead.  */
>> +  append_composite_type_field (type, "_anon_union", sigfault_union_type);

I set the name to "", and it works as you mentioned below.
I also fixed the usages in gdb/testsuite/gdb.base/siginfo-obj.exp

diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index 4660772752d..bbc4009d43c 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -406,9 +406,8 @@ linux_get_siginfo_type (struct gdbarch *gdbarch)
   /* _sigfault is set by SIGILL, SIGFPE, SIGSEGV, SIGBUS, SIGTRAP, SIGEMT */
   type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
   append_composite_type_field (type, "si_addr", void_ptr_type);
-  /* Since there is no possibility to declare an anonymous union,
-     using '_anon_union' instead.  */
-  append_composite_type_field (type, "_anon_union", sigfault_union_type);
+  /* Note: this is an anonymous union.  */
+  append_composite_type_field (type, "", sigfault_union_type);
   append_composite_type_field (sifields_type, "_sigfault", type);

   /* _sigpoll */
diff --git a/gdb/linux-tdep.h b/gdb/linux-tdep.h
index 43ed38c6633..1f40756eb2f 100644
--- a/gdb/linux-tdep.h
+++ b/gdb/linux-tdep.h
@@ -143,14 +143,14 @@ struct siginfo_type

       /* SIGILL, SIGFPE, SIGSEGV, SIGBUS, SIGTRAP, SIGEMT */
       "$_siginfo._sifields._sigfault.si_addr",
-      "$_siginfo._sifields._sigfault._anon_union.si_trapno",
-      "$_siginfo._sifields._sigfault._anon_union.si_addr_lsb",
-      "$_siginfo._sifields._sigfault._anon_union._addr_bnd.si_lower",
-      "$_siginfo._sifields._sigfault._anon_union._addr_bnd.si_upper",
-      "$_siginfo._sifields._sigfault._anon_union._addr_pkey.si_pkey",
-      "$_siginfo._sifields._sigfault._anon_union._perf.si_perf_data",
-      "$_siginfo._sifields._sigfault._anon_union._perf.si_perf_type",
-      "$_siginfo._sifields._sigfault._anon_union._perf.si_perf_flags",
+      "$_siginfo._sifields._sigfault.si_trapno",
+      "$_siginfo._sifields._sigfault.si_addr_lsb",
+      "$_siginfo._sifields._sigfault._addr_bnd.si_lower",
+      "$_siginfo._sifields._sigfault._addr_bnd.si_upper",
+      "$_siginfo._sifields._sigfault._addr_pkey.si_pkey",
+      "$_siginfo._sifields._sigfault._perf.si_perf_data",
+      "$_siginfo._sifields._sigfault._perf.si_perf_type",
+      "$_siginfo._sifields._sigfault._perf.si_perf_flags",
     };
     return paths[static_cast<size_t> (attr_)];
   }
diff --git a/gdb/testsuite/gdb.base/siginfo-obj.exp b/gdb/testsuite/gdb.base/siginfo-obj.exp
index 5e36b334068..a724a634dc0 100644
--- a/gdb/testsuite/gdb.base/siginfo-obj.exp
+++ b/gdb/testsuite/gdb.base/siginfo-obj.exp
@@ -115,7 +115,7 @@ gdb_test "p \$_siginfo._sifields._sigfault.si_addr = 0x666" " = \\(void \\*\\) 0
 gdb_test "p \$_siginfo.si_errno = 666" " = 666"
 gdb_test "p \$_siginfo.si_code = 999" " = 999"
 gdb_test "p \$_siginfo.si_signo = 11" " = 11"
-gdb_test "p \$_siginfo._sifields._sigfault._anon_union._addr_pkey.si_pkey = 123" " = 123"
+gdb_test "p \$_siginfo._sifields._sigfault._addr_pkey.si_pkey = 123" " = 123"

 with_test_prefix "validate modified siginfo fields" {
     gdb_test "break $bp_location"
@@ -143,7 +143,7 @@ if {$gcore_created} {
     gdb_test "p \$_siginfo._sifields._sigfault.si_addr" \
        " = \\(void \\*\\) $ssi_addr" \
        "p \$_siginfo._sifields._sigfault.si_addr from core file"
-    gdb_test "p \$_siginfo._sifields._sigfault._anon_union._addr_pkey.si_pkey" \
+    gdb_test "p \$_siginfo._sifields._sigfault._addr_pkey.si_pkey" \
        " = $ssi_pkey" \
-       "p \$_siginfo._sifields._sigfault._anon_union._addr_pkey.si_pkey from core file"
+       "p \$_siginfo._sifields._sigfault._addr_pkey.si_pkey from core file"
 }


> > Can you expand on why it's not possible to have an anonymous union?  It
> is certainly possible to have anonymous unions described in DWARF, which
> are then translated to struct types.
> 

I simply could not find how to implement it.
It might be useful to add to the documentation of append_composite_type_field_aligned() or maybe
better, to the comment in gdb/gdbtypes.h above the declaration of append_composite_type_field(), a
sentence explaining what is the effect of providing an empty name.

diff --git a/gdb/gdbtypes.h b/gdb/gdbtypes.h
index dd2d24fa8e2..e4fedd886c4 100644
--- a/gdb/gdbtypes.h
+++ b/gdb/gdbtypes.h
@@ -2431,11 +2431,12 @@ extern struct type *init_pointer_type (type_allocator &alloc, int bit,
 extern struct type *init_fixed_point_type (type_allocator &, int, int,
                                           const char *);

-/* Helper functions to construct a struct or record type.  An
-   initially empty type is created using arch_composite_type().
-   Fields are then added using append_composite_type_field*().  A union
-   type has its size set to the largest field.  A struct type has each
-   field packed against the previous.  */
+/* Helper functions to construct a struct or record type.  An initially empty
+   type is created using arch_composite_type().  Fields are then added using
+   append_composite_type_field*().
+   A union type has its size set to the largest field.  A struct type has each
+   field packed against the previous.
+   If no name is specified, the type is anonymous.  */

 extern struct type *arch_composite_type (struct gdbarch *gdbarch,
                                         const char *name, enum type_code code);

> I think that the ideal user experience would be for users to be able to
> access fields the same way that they do in the code, that is
> `si.si_pkey`.  All the _sigfault/_addr_pkey/etc parts are implementation
> details that could change.
> 
> On top of your patch, if I just delete all the internal field names, it
> seems to work just fine, see patch below.  In the end it models
> something like this in C:
> 

I am not against it.
However, could this suggestion be addressed in a different patch ?

Simplifying the existing pathes to si_* values with anonymous structs would increase the impact of
the original patch, with potentially additional testing and carefulness required for others
architectures (for example, see gdb/nat/amd64-linux-siginfo.c L269).

Matthieu

> struct siginto_t
> {
>   union
>   {
>     // kill
>     struct
>     {
>       int si_pid;
>       int si_uid;
>     };
> 
>     // timer
>     struct
>     {
>       int si_tid;
>       int si_overrun;
>       int si_sys_private;
>     };
> 
>     ...
>   };
> };
> 
> 
> diff --git i/gdb/linux-tdep.c w/gdb/linux-tdep.c
> index 4660772752d6..f21fa1e18578 100644
> --- i/gdb/linux-tdep.c
> +++ w/gdb/linux-tdep.c
> @@ -349,21 +349,21 @@ linux_get_siginfo_type (struct gdbarch *gdbarch)
>    type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
>    append_composite_type_field (type, "si_pid", pid_type);
>    append_composite_type_field (type, "si_uid", uid_type);
> -  append_composite_type_field (sifields_type, "_kill", type);
> +  append_composite_type_field (sifields_type, "", type);
> 
>    /* _timer */
>    type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
>    append_composite_type_field (type, "si_tid", int_type);
>    append_composite_type_field (type, "si_overrun", int_type);
>    append_composite_type_field (type, "si_sigval", sigval_type);
> -  append_composite_type_field (sifields_type, "_timer", type);
> +  append_composite_type_field (sifields_type, "", type);
> 
>    /* _rt */
>    type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
>    append_composite_type_field (type, "si_pid", pid_type);
>    append_composite_type_field (type, "si_uid", uid_type);
>    append_composite_type_field (type, "si_sigval", sigval_type);
> -  append_composite_type_field (sifields_type, "_rt", type);
> +  append_composite_type_field (sifields_type, "", type);
> 
>    /* _sigchld */
>    type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
> @@ -372,7 +372,7 @@ linux_get_siginfo_type (struct gdbarch *gdbarch)
>    append_composite_type_field (type, "si_status", int_type);
>    append_composite_type_field (type, "si_utime", clock_type);
>    append_composite_type_field (type, "si_stime", clock_type);
> -  append_composite_type_field (sifields_type, "_sigchld", type);
> +  append_composite_type_field (sifields_type, "", type);
> 
>    /* Begin _sigfault's anonymous union.  */
>    sigfault_union_type = arch_composite_type (gdbarch, NULL, TYPE_CODE_UNION);
> @@ -386,20 +386,20 @@ linux_get_siginfo_type (struct gdbarch *gdbarch)
>    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);
> +  append_composite_type_field (sigfault_union_type, "", type);
> 
>    /* used when si_code=SEGV_PKUERR */
>    type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
>    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);
> +  append_composite_type_field (sigfault_union_type, "", type);
> 
>    /* used when si_code=TRAP_PERF */
>    type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
>    append_composite_type_field (type, "si_perf_data", unsigned_long_type);
>    append_composite_type_field (type, "si_perf_type", uint32_type);
>    append_composite_type_field (type, "si_perf_flags", uint32_type);
> -  append_composite_type_field (sigfault_union_type, "_perf", type);
> +  append_composite_type_field (sigfault_union_type, "", type);
> 
>    /* End _sigfault's anonymous union.  */
> 
> @@ -408,21 +408,21 @@ linux_get_siginfo_type (struct gdbarch *gdbarch)
>    append_composite_type_field (type, "si_addr", void_ptr_type);
>    /* Since there is no possibility to declare an anonymous union,
>       using '_anon_union' instead.  */
> -  append_composite_type_field (type, "_anon_union", sigfault_union_type);
> -  append_composite_type_field (sifields_type, "_sigfault", type);
> +  append_composite_type_field (type, "", sigfault_union_type);
> +  append_composite_type_field (sifields_type, "", type);
> 
>    /* _sigpoll */
>    type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
>    append_composite_type_field (type, "si_band", long_type);
>    append_composite_type_field (type, "si_fd", int_type);
> -  append_composite_type_field (sifields_type, "_sigpoll", type);
> +  append_composite_type_field (sifields_type, "", type);
> 
>    /* _sigsys */
>    type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
>    append_composite_type_field (type, "_call_addr", void_ptr_type);
>    append_composite_type_field (type, "_syscall", int_type);
>    append_composite_type_field (type, "_arch", uint_type);
> -  append_composite_type_field (sifields_type, "_sigsys", type);
> +  append_composite_type_field (sifields_type, "", type);
> 
>    /* struct siginfo */
>    siginfo_type = arch_composite_type (gdbarch, NULL, TYPE_CODE_STRUCT);
> @@ -431,7 +431,7 @@ linux_get_siginfo_type (struct gdbarch *gdbarch)
>    append_composite_type_field (siginfo_type, "si_errno", int_type);
>    append_composite_type_field (siginfo_type, "si_code", int_type);
>    append_composite_type_field_aligned (siginfo_type,
> -				       "_sifields", sifields_type,
> +				       "", sifields_type,
>  				       long_type->length ());
> 
>    linux_gdbarch_data->siginfo_type = siginfo_type;
> diff --git i/gdb/testsuite/gdb.base/siginfo-obj.exp w/gdb/testsuite/gdb.base/siginfo-obj.exp
> index 5e36b3340680..272d74ac2805 100644
> --- i/gdb/testsuite/gdb.base/siginfo-obj.exp
> +++ w/gdb/testsuite/gdb.base/siginfo-obj.exp
> @@ -111,11 +111,11 @@ gdb_test "continue" ".*Program received signal SIGSEGV.*" \
>  	 "continue to signal, 2nd"
> 
>  set test "set si_addr"
> -gdb_test "p \$_siginfo._sifields._sigfault.si_addr = 0x666" " = \\(void \\*\\) 0x666"
> +gdb_test "p \$_siginfo.si_addr = 0x666" " = \\(void \\*\\) 0x666"
>  gdb_test "p \$_siginfo.si_errno = 666" " = 666"
>  gdb_test "p \$_siginfo.si_code = 999" " = 999"
>  gdb_test "p \$_siginfo.si_signo = 11" " = 11"
> -gdb_test "p \$_siginfo._sifields._sigfault._anon_union._addr_pkey.si_pkey = 123" " = 123"
> +gdb_test "p \$_siginfo.si_pkey = 123" " = 123"
> 
>  with_test_prefix "validate modified siginfo fields" {
>      gdb_test "break $bp_location"
> @@ -140,10 +140,10 @@ if {$gcore_created} {
>  	"p \$_siginfo.si_errno from core file"
>      gdb_test "p \$_siginfo.si_code" " = $ssi_code" \
>  	"p \$_siginfo.si_code from core file"
> -    gdb_test "p \$_siginfo._sifields._sigfault.si_addr" \
> +    gdb_test "p \$_siginfo.si_addr" \
>  	" = \\(void \\*\\) $ssi_addr" \
> -	"p \$_siginfo._sifields._sigfault.si_addr from core file"
> -    gdb_test "p \$_siginfo._sifields._sigfault._anon_union._addr_pkey.si_pkey" \
> +	"p \$_siginfo.si_addr from core file"
> +    gdb_test "p \$_siginfo.si_pkey" \
>  	" = $ssi_pkey" \
> -	"p \$_siginfo._sifields._sigfault._anon_union._addr_pkey.si_pkey from core file"
> +	"p \$_siginfo.si_pkey from core file"
>  }
> 
> Simon