Re: [PATCH 1/2] xen/build-id: Introduce is_gnu_build_id()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.11.2026 14:51, Andrew Cooper wrote:
> Broken out of the subsequent change for legibility reasons.  It will be used
> to replace xen_build_id_check().
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> CC: Michal Orzel <michal.orzel@xxxxxxx>
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Julien Grall <julien@xxxxxxx>
> CC: Roger Pau Monné <roger@xxxxxxxxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> ---
>  xen/common/version.c         | 33 +++++++++++++++++++++++++++++++++
>  xen/include/xen/elfstructs.h |  2 ++
>  xen/include/xen/version.h    |  2 ++
>  3 files changed, 37 insertions(+)
> 
> diff --git a/xen/common/version.c b/xen/common/version.c
> index 80e7a9b981cf..7eb8a0221d82 100644
> --- a/xen/common/version.c
> +++ b/xen/common/version.c
> @@ -114,6 +114,37 @@ void print_version(void)
>  /* Defined in linker script. */
>  extern const Elf_Note __note_gnu_build_id_start[], __note_gnu_build_id_end[];
>  
> +/*
> + * Check whether @n is a GNU Build ID note.
> + *
> + * We check that:
> + *  - type is NT_GNU_BUILD_ID
> + *  - name is GNU\0
> + *  - desc is non-empty and exactly covering the rest of the buffer
> + *
> + * If this returns true, the contents starts at n + NT_GNU_HDR_SIZE.
> + */
> +bool is_gnu_build_id(const Elf_Note *n, unsigned int len)
> +{
> +    const char *name;
> +
> +    if ( len <= NT_GNU_HDR_SIZE )
> +        return false;
> +
> +    if ( n->type != NT_GNU_BUILD_ID ||
> +         n->namesz != 4 )
> +        return false;
> +
> +    name = ELFNOTE_NAME(n);
> +    if ( strncmp(name, "GNU", 4) != 0 )
> +        return false;
> +
> +    if ( len != n->descsz + NT_GNU_HDR_SIZE )
> +        return false;

Isn't this too strict a check? Build-IDs can, iirc, take different formats
(yes, this is under our control, but still), and I don't think there's a
guarantee that all current and future ones are coming without any padding.
IOW I think len wants comparing against the rounded up sum.

Jan