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SubjectRe: [PATCH 09/11] kexec_file: Remove mis-use of sh_offset field
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@linux.vnet.ibm.com>
> ---
> 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

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