Re: [PATCH 2/7] x86/mkelf32: account for offset when detecting note segment placement

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.03.2025 18:35, Roger Pau Monne wrote:
> mkelf32 attempt to check that the program header defined NOTE segment falls
> inside of the LOAD segment, as the build-id should be loaded for Xen at
> runtime to check.
> 
> However the current code doesn't take into account the LOAD program header
> segment offset when calculating overlap with the NOTE segment.  This
> results in incorrect detection, and the following build error:
> 
> arch/x86/boot/mkelf32 --notes xen-syms ./.xen.elf32 0x200000 \
>                `nm xen-syms | sed -ne 's/^\([^ ]*\) . 
> __2M_rwdata_end$/0x\1/p'`
> Expected .note section within .text section!
> Offset 4244776 not within 2910364!

Not your fault, but: Such printing of decimal numbers is of course very
unhelpful when ...

> When xen-syms has the following program headers:
> 
> Program Header:
>     LOAD off    0x0000000000200000 vaddr 0xffff82d040200000 paddr 
> 0x0000000000200000 align 2**21
>          filesz 0x00000000002c689c memsz 0x00000000003f7e20 flags rwx
>     NOTE off    0x000000000040c528 vaddr 0xffff82d04040c528 paddr 
> 0x000000000040c528 align 2**2
>          filesz 0x0000000000000024 memsz 0x0000000000000024 flags r--

... any half-way sane tool prints such values in hex.

> --- a/xen/arch/x86/boot/mkelf32.c
> +++ b/xen/arch/x86/boot/mkelf32.c
> @@ -358,7 +358,8 @@ int main(int argc, char **argv)
>          note_sz = in64_phdr.p_memsz;
>          note_base = in64_phdr.p_vaddr - note_base;
>  
> -        if ( in64_phdr.p_offset > dat_siz || offset > in64_phdr.p_offset )
> +        if ( in64_phdr.p_offset > (offset + dat_siz) ||
> +             offset > in64_phdr.p_offset )

This is an improvement, so fine to go in with Andrew's ack, but it still
doesn't match what the error message suggests is wanted: This checks only
whether .note starts after .text or ends before .text. A partial overlap,
which isn't okay either aiui, would go through fine.

Oh, and - even in your change there's an off-by-1: You mean >= in the lhs
of the ||.

Jan