Re: [PATCH 09/11] kexec_file: Remove mis-use of sh_offset field

[Dave Young]

On 02/26/18 at 04:16pm, Philipp Rudo wrote:
> The current code uses the sh_offset field in purgatory_info->sechdrs to
> store a pointer to the current load address of the section. Depending
> whether the section will be loaded or not this is either a pointer into
> purgatory_info->purgatory_buf or kexec_purgatory. This is not only a
> violation of the ELF standard but also makes the code very hard to
> understand as you cannot tell if the memory you are using is read-only or
> not.
> 
> Remove this mis-use and store the offset of the section in
> pugaroty_info->purgatory_buf in sh_offset.
> 
> Signed-off-by: Philipp Rudo <prudo@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/machine_kexec_64.c | 10 ++++++----
>  kernel/kexec_file.c                | 33 +++------------------------------
>  2 files changed, 9 insertions(+), 34 deletions(-)
> 
> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index 51667c8b5c9b..41db74bdc88b 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -457,13 +457,15 @@ int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
>  		 * rel[i].r_offset contains byte offset from beginning
>  		 * of section to the storage unit affected.
>  		 *
> -		 * This is location to update (->sh_offset). This is temporary
> -		 * buffer where section is currently loaded. This will finally
> -		 * be loaded to a different address later, pointed to by
> +		 * This is location to update. This is temporary buffer
> +		 * where section is currently loaded. This will finally be
> +		 * loaded to a different address later, pointed to by
>  		 * ->sh_addr. kexec takes care of moving it
>  		 *  (kexec_load_segment()).
>  		 */
> -		location = (void *)(section->sh_offset + rel[i].r_offset);
> +		location = pi->purgatory_buf;
> +		location += section->sh_offset;
> +		location += rel[i].r_offset;
>  
>  		/* Final address of the location */
>  		address = section->sh_addr + rel[i].r_offset;
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index 746b91e46e34..25b44d1a664a 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -736,28 +736,6 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
>  	       pi->ehdr->e_shnum * sizeof(Elf_Shdr));
>  	pi->sechdrs = sechdrs;
>  
> -	/*
> -	 * We seem to have multiple copies of sections. First copy is which
> -	 * is embedded in kernel in read only section. Some of these sections
> -	 * will be copied to a temporary buffer and relocated. And these
> -	 * sections will finally be copied to their final destination at
> -	 * segment load time.

It would be good to keep above part comment somewhere..

> -	 *
> -	 * Use ->sh_offset to reflect section address in memory. It will
> -	 * point to original read only copy if section is not allocatable.
> -	 * Otherwise it will point to temporary copy which will be relocated.
> -	 *
> -	 * Use ->sh_addr to contain final address of the section where it
> -	 * will go during execution time.
> -	 */
> -	for (i = 0; i < pi->ehdr->e_shnum; i++) {
> -		if (sechdrs[i].sh_type == SHT_NOBITS)
> -			continue;
> -
> -		sechdrs[i].sh_offset = (unsigned long)pi->ehdr +
> -						sechdrs[i].sh_offset;
> -	}
> -
>  	offset = 0;
>  	bss_addr = kbuf->mem + kbuf->bufsz;
>  	kbuf->image->start = pi->ehdr->e_entry;
> @@ -786,17 +764,12 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
>  			kbuf->image->start += kbuf->mem + offset;
>  		}
>  
> -		src = (void *)sechdrs[i].sh_offset;
> +		src = (void *)pi->ehdr + sechdrs[i].sh_offset;
>  		dst = pi->purgatory_buf + offset;
>  		memcpy(dst, src, sechdrs[i].sh_size);
>  
>  		sechdrs[i].sh_addr = kbuf->mem + offset;
> -
> -		/*
> -		 * This section got copied to temporary buffer. Update
> -		 * ->sh_offset accordingly.
> -		 */
> -		sechdrs[i].sh_offset = (unsigned long)dst;
> +		sechdrs[i].sh_offset = offset;
>  		offset += sechdrs[i].sh_size;
>  	}
>  
> @@ -1006,7 +979,7 @@ int kexec_purgatory_get_set_symbol(struct kimage *image, const char *name,
>  		return -EINVAL;
>  	}
>  
> -	sym_buf = (char *)sec->sh_offset + sym->st_value;
> +	sym_buf = (char *)pi->purgatory_buf + sec->sh_offset + sym->st_value;
>  
>  	if (get_value)
>  		memcpy((void *)buf, sym_buf, size);
> -- 
> 2.13.5
> 

Thanks
Dave