Re: [PATCH v3 07/14] dump: Swap segment and section header locations

[Marc-André Lureau]

Hi

On Thu, Jul 21, 2022 at 5:23 PM Janosch Frank <frankja@linux.ibm.com> wrote:
>
> For the upcoming string table and arch section support we need to
> modify the elf layout a bit. Instead of the segments, i.e. the guest's
> memory contents, beeing the last area the section data will live at
> the end of the file. This will allow us to write the section data
> after all guest memory has been dumped which is important for the s390
> PV dump support.
>
> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
> ---
>  dump/dump.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
>
> diff --git a/dump/dump.c b/dump/dump.c
> index 980702d476..6f3274c5af 100644
> --- a/dump/dump.c
> +++ b/dump/dump.c
> @@ -590,6 +590,9 @@ static void dump_begin(DumpState *s, Error **errp)
>       *   --------------
>       *   |  memory     |
>       *   --------------
> +     *   |  sectn data |
> +     *   --------------
> +
>       *
>       * we only know where the memory is saved after we write elf note into
>       * vmcore.
> @@ -1817,18 +1820,18 @@ static void dump_init(DumpState *s, int fd, bool 
> has_format,
>          }
>      }
>
> +    tmp = (s->phdr_num == PN_XNUM) ? s->sh_info : s->phdr_num;
>      if (dump_is_64bit(s)) {
> -        s->phdr_offset = sizeof(Elf64_Ehdr);
> -        s->shdr_offset = s->phdr_offset + sizeof(Elf64_Phdr) * s->phdr_num;
> -        s->note_offset = s->shdr_offset + sizeof(Elf64_Shdr) * s->shdr_num;
> -        s->memory_offset = s->note_offset + s->note_size;
> +        s->shdr_offset = sizeof(Elf64_Ehdr);
> +        s->phdr_offset = s->shdr_offset + sizeof(Elf64_Shdr) * s->shdr_num;
> +        s->note_offset = s->phdr_offset + sizeof(Elf64_Phdr) * tmp;
>      } else {
> -
> -        s->phdr_offset = sizeof(Elf32_Ehdr);
> -        s->shdr_offset = s->phdr_offset + sizeof(Elf32_Phdr) * s->phdr_num;
> -        s->note_offset = s->shdr_offset + sizeof(Elf32_Shdr) * s->shdr_num;
> -        s->memory_offset = s->note_offset + s->note_size;
> +        s->shdr_offset = sizeof(Elf32_Ehdr);
> +        s->phdr_offset = s->shdr_offset + sizeof(Elf32_Shdr) * s->shdr_num;
> +        s->note_offset = s->phdr_offset + sizeof(Elf32_Phdr) * tmp;
>      }
> +    s->memory_offset = s->note_offset + s->note_size;
> +    s->section_offset = s->memory_offset + s->total_size;

Ah! section_offset was actually added from commit e71d353360 ("dump:
Add more offset variables"), but unused until now. It's worth
explaining in the commit message.

>
>      return;
>
> --
> 2.34.1
>