Re: [PATCH v3 1/2] powerpc/crash: fix backup region offset update to elfcorehdr

[Hari Bathini <hbathini@linux.ibm.com>]

On 12/03/26 2:00 pm, Sourabh Jain wrote:
> update_backup_region_phdr() in file_load_64.c iterates over all the
> program headers in the kdump kernel’s elfcorehdr and updates the
> p_offset of the program header whose physical address starts at 0.
> 
> However, the loop logic is incorrect because the program header pointer
> is not updated during iteration. Since elfcorehdr typically contains
> PT_NOTE entries first, the PT_LOAD program header with physical address
> 0 is never reached. As a result, its p_offset is not updated to point to
> the backup region.
> 
> Because of this behavior, the capture kernel exports the first 64 KB of
> the crashed kernel’s memory at offset 0, even though that memory
> actually lives in the backup region. When a crash happens, purgatory
> copies the first 64 KB of the crashed kernel’s memory into the backup
> region so the capture kernel can safely use it.
> 
> This has not caused problems so far because the first 64 KB is usually
> identical in both the crashed and capture kernels. However, this is
> just an assumption and is not guaranteed to always hold true.
> 
> Fix update_backup_region_phdr() to correctly update the p_offset of the
> program header with a starting physical address of 0 by correcting the
> logic used to iterate over the program headers.
> 

Thanks for fixing this.
Looks good to me.

Reviewed-by: Hari Bathini <hbathini@linux.ibm.com>

> Fixes: cb350c1f1f86 ("powerpc/kexec_file: Prepare elfcore header for crashing kernel")
> Cc: Hari Bathini <hbathini@linux.ibm.com>
> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
> Reviewed-by: Aditya Gupta <adityag@linux.ibm.com>
> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> ---
>   arch/powerpc/kexec/file_load_64.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kexec/file_load_64.c b/arch/powerpc/kexec/file_load_64.c
> index e7ef8b2a2554..e631cf2eda2c 100644
> --- a/arch/powerpc/kexec/file_load_64.c
> +++ b/arch/powerpc/kexec/file_load_64.c
> @@ -391,7 +391,7 @@ static void update_backup_region_phdr(struct kimage *image, Elf64_Ehdr *ehdr)
>   	unsigned int i;
>   
>   	phdr = (Elf64_Phdr *)(ehdr + 1);
> -	for (i = 0; i < ehdr->e_phnum; i++) {
> +	for (i = 0; i < ehdr->e_phnum; i++, phdr++) {
>   		if (phdr->p_paddr == BACKUP_SRC_START) {
>   			phdr->p_offset = image->arch.backup_start;
>   			kexec_dprintk("Backup region offset updated to 0x%lx\n",