[PATCH] PE/COFF: auto-promote to bigobj target when section count overflows

[Jan Beulich]

On 21.06.2026 19:49, Oleg Tolmatcev wrote:
> Regular PE/COFF uses 16-bit section numbers in the symbol table,
> limiting objects to at most 65535 sections.  When this limit is
> exceeded, the assembler silently truncates section numbers, producing
> corrupt objects that cause confusing linker errors like:
> 
>   undefined reference to `vtable for ...'
> 
> Fix this by automatically switching the xvec to the matching bigobj
> target before any headers or symbols are finalized, so the object is
> written out using the extended COFF format which supports 32-bit
> section numbers.

It has never really become clear to me why this wouldn't have been the
default behavior from the very beginning. Perhaps with a way to opt out
(rather than one to opt in).

> --- a/bfd/coffcode.h
> +++ b/bfd/coffcode.h
> @@ -360,6 +360,10 @@ CODE_FRAGMENT
>  #include "coffswap.h"
>  #endif
>  
> +#ifdef COFF_WITH_PE_BIGOBJ
> +extern const bfd_target TARGET_SYM_BIG;
> +#endif
> +
>  #define STRING_SIZE_SIZE 4
>  
>  #define DOT_DEBUG	".debug"
> @@ -3154,6 +3158,16 @@ coff_compute_section_file_positions (bfd * abfd)
>  
>    if (target_index >= bfd_coff_max_nscns (abfd))
>      {
> +#ifdef COFF_WITH_PE_BIGOBJ
> +      if (abfd->xvec != &TARGET_SYM_BIG)
> +	{
> +	  /* Regular PE/COFF uses 16-bit section numbers in symbols.
> +	     Promote oversized objects to the matching bigobj target
> +	     before any headers or symbols are finalized.  */
> +	  abfd->xvec = &TARGET_SYM_BIG;
> +	  return coff_compute_section_file_positions (abfd);
> +	}
> +#endif
>        bfd_set_error (bfd_error_file_too_big);
>        _bfd_error_handler
>  	/* xgettext:c-format */

In the description you mention silent generation of a corrupt output. Yet then
you place your addition next to the emission of an error. If this error path
wasn't taken in the case of interest, your new addition also would take any
effect. What am I missing?

> --- /dev/null
> +++ b/gas/testsuite/gas/pe/big-obj-auto.d
> @@ -0,0 +1,9 @@
> +#objdump: -h
> +#name: PE big obj auto-promotion
> +
> +.*: *file format pe-bigobj-.*
> +
> +Sections:
> +#...
> +5000. \.data\$a49999  .*

5000. still fits in 16 bits. How is this sufficient a test for the new behavior?
I expect you want ...

> --- /dev/null
> +++ b/gas/testsuite/gas/pe/big-obj-auto.s
> @@ -0,0 +1,16 @@
> +	.file "big-obj-auto.s"
> +
> +	.irp n,0,1,2,3,4

	.irp n,0,1,2,3,4,5,6,7

here.

> +	.irp m,0,1,2,3,4,5,6,7,8,9
> +	.irp c,0,1,2,3,4,5,6,7,8,9
> +	.irp d,0,1,2,3,4,5,6,7,8,9
> +	.irp u,0,1,2,3,4,5,6,7,8,9
> +	.globl a\n\m\c\d\u
> +	.section .data$a\n\m\c\d\u,"w"
> +a\n\m\c\d\u :
> +	.byte 1
> +	.endr
> +	.endr
> +	.endr
> +	.endr
> +	.endr
> --- a/gas/testsuite/gas/pe/pe.exp
> +++ b/gas/testsuite/gas/pe/pe.exp
> @@ -70,4 +70,5 @@ if {[istarget "aarch64-*-pe*"] || [istarget "aarch64-*-mingw*"]} {
>  
>  if ([istarget "*-*-mingw*"]) then {
>  	run_dump_test "big-obj"
> +	run_dump_test "big-obj-auto"
>  }

Why would the test want limiting to MinGW?

Jan