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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.03.2025 15:16, Roger Pau Monné wrote:
> On Wed, Mar 19, 2025 at 11:07:33AM +0100, Jan Beulich wrote:
>> 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 ||.
> 
> Hm, I see, it would be better as:
> 
> diff --git a/xen/arch/x86/boot/mkelf32.c b/xen/arch/x86/boot/mkelf32.c
> index 5f9e7e440e84..f0f406687cea 100644
> --- a/xen/arch/x86/boot/mkelf32.c
> +++ b/xen/arch/x86/boot/mkelf32.c
> @@ -358,11 +358,13 @@ 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 ||
> +             in64_phdr.p_offset + in64_phdr.p_filesz > offset + dat_siz )
>          {
>              fprintf(stderr, "Expected .note section within .text section!\n" 
> \
> -                    "Offset %"PRId64" not within %d!\n",
> -                    in64_phdr.p_offset, dat_siz);
> +                    ".note: [%"PRIx64", %"PRIx64") .text: [%x, %x)\n",
> +                    in64_phdr.p_offset, in64_phdr.p_offset + 
> in64_phdr.p_filesz,
> +                    offset, offset + dat_siz);
>              return 1;
>          }
>          /* Gets us the absolute offset within the .text section. */

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>