[PATCH v2] Add support for recording minimal symbols in AIX for non DWARF compiled library function record.

[PATCH v2] Add support for recording minimal symbols in AIX for non DWARF compiled library function record.

[Aditya Vidyadhar Kamath]

From: Aditya Vidyadhar Kamath <aditya.kamath1@ibm.com>

Commit 1dfd89c739 (gdb: Remove stabs support from XCOFF inferiors) removed STABS handling from xcoffread.c.
While removing full symtab and compunit_symtab construction is appropriate when DWARF is available,
the change also removed recording of minimal symbols, which still provide value for XCOFF binaries that lack DWARF.

On AIX, system libraries such as libc and libpthread are built with STABS debug information up to AIX 7.3 TL4.
As a result, current GDB master is unable to record even minimal symbols for these libraries, leading to missing function names (e.g. printf) in common debugging scenarios.

Consider the example below,
int g_in_lib = 777;

int lib_func() {
        return g_in_lib + 1;
}

int lib_func();

int main() {
        printf ("lib_func() = %d \n", lib_func());
        return 0;
}

We build libx and then link it to main.

When we compile this in GDB in AIX today in master branch we get,

Reading symbols from //gdb_tests/main...
(gdb) b main
Breakpoint 1 at 0x10000530: file //gdb_tests/main.c, line 5.
(gdb) r
Starting program: /gdb_tests/main

Breakpoint 1, main () at //gdb_tests/main.c:5
5           printf ("lib_func() = %d \n", lib_func());
(gdb) call printf()
No symbol "printf" in current context.
(gdb)

The issue is Without minimal symbols:
 - Functions from system libraries are not visible in backtraces.
 - Calls such as call printf() fail with no symbol in current context.
 - This affects not only stepping, but also basic usability of backtraces on AIX systems without DWARF-enabled system libraries.
 - Even when full debug information is unavailable, minimal symbols are still sufficient to:
 - Identify function entry/exit points.
 - Display meaningful backtraces involving system libraries.

This patch restores recording of minimal symbols in xcoffread.c, while keeping the removal of:
 - STABS-based symtabs
 - compunit_symtabs

The below function (read_xcoff_minimal_symbols) ensures that:
 - record_minimal_symbol() is still invoked where appropriate.
 - No attempt is made to rebuild full debug information from STABS.
 - DWARF remains the sole source for full debugging information when available.
This keeps the original intent of the STABS removal intact, while restoring essential functionality for non-DWARF XCOFF binaries.

After applying the patch we get,
GNU gdb (GDB) 18.0.50.20260316-git
Copyright (C) 2026 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "powerpc64-ibm-aix7.2.0.0".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.

+------------------------------------------------------------------------------+
| Find the GDB manual online at:                                               |
| http://www.gnu.org/software/gdb/documentation/.                              |
| For help, type "help".                                                       |
| Type "apropos word" to search for commands related to "word".                |
+------------------------------------------------------------------------------+

Reading symbols from //gdb_tests/main...
(gdb) b main
Breakpoint 1 at 0x10000530: file //gdb_tests/main.c, line 5.
(gdb) n
The program is not being run.
(gdb) r
Starting program: /gdb_tests/main

Breakpoint 1, main () at //gdb_tests/main.c:5
5           printf ("lib_func() = %d \n", lib_func());
(gdb) call (int)printf ("lib_func() = %d \n", lib_func())
lib_func() = 778
$1 = 18
(gdb)
---
 gdb/xcoffread.c | 91 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)

diff --git a/gdb/xcoffread.c b/gdb/xcoffread.c
index aec959e1375..d02c9294be6 100644
--- a/gdb/xcoffread.c
+++ b/gdb/xcoffread.c
@@ -28,6 +28,7 @@
 #include "xcoffread.h"
 
 #include "symtab.h"
+#include "minsyms.h"
 #include "gdbtypes.h"
 #include "symfile.h"
 #include "objfiles.h"
@@ -232,6 +233,91 @@ xcoff_get_toc_offset (objfile *objfile)
   return 0;
 }
 
+/* This function is used to read minimal sysmbols from the XCOFF STABS
+   symbol table. This function needs to be called when a library (like
+   libc) in AIX is compiled using xlc whose debug format is STABS.
+   When STABS debug information is not available, then in the backtrace
+   or other GDB features the STABS compiled functions like (printf)
+   will not be seen. Though we no longer support STABS debug, this
+   function records minimal symbol or information needed to show
+   such functions in backtrace.  */
+
+static void
+read_xcoff_minimal_symbols (objfile *objfile)
+{
+  bfd *abfd = objfile->obfd.get ();
+  long do_we_need_storage = bfd_get_symtab_upper_bound (abfd);
+
+  if (do_we_need_storage <= 0)
+    return;
+
+  gdb::unique_xmalloc_ptr<asymbol *> sym_table
+    ((asymbol **) xmalloc (do_we_need_storage));
+  /* Get the number of symbols we need.  */
+  int number_of_symbols = bfd_canonicalize_symtab (abfd, sym_table.get());
+
+  /* Return on error.  */
+  if (number_of_symbols < 0)
+      return;
+
+  minimal_symbol_reader reader (objfile);
+  for (int i = 0; i < number_of_symbols; i++)
+    {
+      asymbol *sym = sym_table.get ()[i];
+      flagword sym_flags = sym->flags;
+      CORE_ADDR sym_value = bfd_asymbol_value (sym);
+      asection *sym_section = bfd_asymbol_section (sym);
+      enum minimal_symbol_type ms_type = mst_unknown;
+      const char *sym_name = bfd_asymbol_name (sym);
+
+      /* Skip undefined sections if any.  */
+      if (sym_section == bfd_und_section_ptr)
+	continue;
+
+      /* Skip undefined, special symbols and debugging symbols.  */
+      if (sym_flags & (BSF_DEBUGGING | BSF_SECTION_SYM))
+	continue;
+
+      /* We need to pass ms_type when we record. So find the symbol type.  */
+
+      /* If this is an object file.  */
+      if (sym_flags & BSF_OBJECT)
+	{
+	  /* Code section.  */
+	  if (sym_section && (bfd_section_flags (sym_section) & SEC_CODE))
+	    ms_type = mst_text;
+	  /* Data section.  */
+	  else if (sym_section && (bfd_section_flags (sym_section) & SEC_DATA))
+	    ms_type = mst_data;
+	  /* BSS section.  */
+	  else if (sym_section && (bfd_section_flags (sym_section) & SEC_ALLOC))
+	    ms_type = mst_bss;
+	  else
+	    ms_type = mst_unknown;
+	}
+      /* Check for symbols marked as functions explicitly.  */
+      else if (sym_flags & BSF_FUNCTION)
+	  ms_type = mst_text;
+      /* Section Based Guess. Code or Data or BSS. */
+      else if (sym_section)
+	{
+	  if (bfd_section_flags (sym_section) & SEC_CODE)
+	    ms_type = mst_text;
+	  else if (bfd_section_flags (sym_section) & SEC_DATA)
+	    ms_type = mst_data;
+	  else if (bfd_section_flags (sym_section) & SEC_ALLOC)
+	    ms_type = mst_bss;
+	}
+
+      /* Add the symbol if it's global or weak */
+      if ((sym_flags & (BSF_GLOBAL | BSF_WEAK)) && sym_name && *sym_name)
+	reader.record_with_info (sym_name, unrelocated_addr (sym_value), ms_type,
+				   gdb_bfd_section_index (abfd, sym_section));
+    }
+
+  reader.install ();
+}
+
 /* Read the XCOFF symbol table.  The only thing we are interested in is the TOC
    offset value.  */
 
@@ -240,6 +326,11 @@ xcoff_symfile_read (objfile *objfile, symfile_add_flags symfile_flags)
 {
   xcoff_find_toc_offset (objfile);
 
+  /* Read minimal symbols from the STABS symbol table of an XCOFF binary
+     This makes sure we see functions from libc given libraries link to
+     it and libc in AIX may not be DWARF compiled. . */
+  read_xcoff_minimal_symbols (objfile);
+
   /* DWARF2 sections.  */
   dwarf2_initialize_objfile (objfile, &dwarf2_xcoff_names);
 }
-- 
2.41.0

Re: [PATCH v2] Add support for recording minimal symbols in AIX for non DWARF compiled library function record.

[Tom Tromey]

>>>>> Aditya Vidyadhar Kamath <akamath996@gmail.com> writes:

> This patch restores recording of minimal symbols in xcoffread.c, while keeping the removal of:
>  - STABS-based symtabs
>  - compunit_symtabs

Thanks for updating the patch.

I'm sorry I didn't notice / remember this before, but there is one more
change that should be made.

> +  bfd *abfd = objfile->obfd.get ();
> +  long do_we_need_storage = bfd_get_symtab_upper_bound (abfd);
> +
> +  if (do_we_need_storage <= 0)
> +    return;
> +
> +  gdb::unique_xmalloc_ptr<asymbol *> sym_table
> +    ((asymbol **) xmalloc (do_we_need_storage));
> +  /* Get the number of symbols we need.  */
> +  int number_of_symbols = bfd_canonicalize_symtab (abfd, sym_table.get());
> +
> +  /* Return on error.  */
> +  if (number_of_symbols < 0)
> +      return;

This code should be replaced with a call to gdb_bfd_canonicalize_symtab.
That was added to avoid some pathological behavior, see the comment in
gdb_bfd.h and/or the associated bug.

Since you don't want an error here you should pass 'false' for
should_throw.

I forgot about the existence of this earlier and only remembered it when
I was looking at something else...

thanks,
Tom

RE: [PATCH v2] Add support for recording minimal symbols in AIX for non DWARF compiled library function record.

[Aditya Kamath]

[-- Attachment #1: Type: text/plain, Size: 1107 bytes --]

Hi Tom,

Thank you for your feedback. I have sent a v3 of this patch with the suggestions.

>> +  bfd *abfd = objfile->obfd.get ();
>> +  long do_we_need_storage = bfd_get_symtab_upper_bound (abfd);
>> +
>> +  if (do_we_need_storage <= 0)
>> +    return;
>> +
>> +  gdb::unique_xmalloc_ptr<asymbol *> sym_table
>> +    ((asymbol **) xmalloc (do_we_need_storage));
>> +  /* Get the number of symbols we need.  */
>> +  int number_of_symbols = bfd_canonicalize_symtab (abfd, sym_table.get());
>> +
>> +  /* Return on error.  */
>> +  if (number_of_symbols < 0)
>> +      return;

>This code should be replaced with a call to >gdb_bfd_canonicalize_symtab.
>That was added to avoid some pathological behavior, see the >?>comment in
>gdb_bfd.h and/or the associated bug.

>Since you don't want an error here you should pass 'false’ for
>should_throw.

>I forgot about the existence of this earlier and only remembered it >when
>I was looking at something else…

Yeah, it is a nice idea and glad you suggested it.

Thanks once again.

Have a nice day ahead.

Regards,
Aditya.

[-- Attachment #2: Type: text/html, Size: 2865 bytes --]