Re: [PATCH] linux-user: Fix loading of BSS segments

[Laurent Vivier]

Le 17/12/2020 à 11:17, Giuseppe Musacchio a écrit :
> Some ELF binaries encode the .bss section as an extension of the data
> ones by setting the segment p_memsz > p_filesz. Some other binaries take
> a different route and encode it as a stand-alone PT_LOAD segment with
> p_filesz = 0 and p_memsz > 0.
> 
> Both the encodings are actually correct per ELF specification but the
> ELF loader had some troubles in handling the former: with the old logic
> it was very likely to get Qemu to crash in zero_bss when trying to
> access unmapped memory.
> 
> zero_bss isn't meant to allocate whole zero-filled segments but to
> "complete" a previously mapped segment with the needed zero bits.
> 
> The fix is pretty simple, if the segment is completely zero-filled we
> simply allocate one or more pages (according to p_memsz) and avoid
> calling zero_bss altogether.
> 
> Signed-off-by: Giuseppe Musacchio <thatlemon@gmail.com>
> ---
>  linux-user/elfload.c | 30 ++++++++++++++++++++----------
>  1 file changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> index 0b02a92602..a16c240e0f 100644
> --- a/linux-user/elfload.c
> +++ b/linux-user/elfload.c
> @@ -2776,14 +2776,16 @@ static void load_elf_image(const char *image_name, 
> int image_fd,
>              vaddr = load_bias + eppnt->p_vaddr;
>              vaddr_po = TARGET_ELF_PAGEOFFSET(vaddr);
>              vaddr_ps = TARGET_ELF_PAGESTART(vaddr);
> -            vaddr_len = TARGET_ELF_PAGELENGTH(eppnt->p_filesz + vaddr_po);
> +
> +            vaddr_ef = vaddr + eppnt->p_filesz;
> +            vaddr_em = vaddr + eppnt->p_memsz;
>  
>              /*
> -             * Some segments may be completely empty without any backing file
> -             * segment, in that case just let zero_bss allocate an empty 
> buffer
> -             * for it.
> +             * Some segments may be completely empty, with a non-zero p_memsz
> +             * but no backing file segment.
>               */
>              if (eppnt->p_filesz != 0) {
> +                vaddr_len = TARGET_ELF_PAGELENGTH(eppnt->p_filesz + 
> vaddr_po);
>                  error = target_mmap(vaddr_ps, vaddr_len, elf_prot,
>                                      MAP_PRIVATE | MAP_FIXED,
>                                      image_fd, eppnt->p_offset - vaddr_po);
> @@ -2791,14 +2793,22 @@ static void load_elf_image(const char *image_name, 
> int image_fd,
>                  if (error == -1) {
>                      goto exit_mmap;
>                  }
> -            }
>  
> -            vaddr_ef = vaddr + eppnt->p_filesz;
> -            vaddr_em = vaddr + eppnt->p_memsz;
> +                /*
> +                 * If the load segment requests extra zeros (e.g. bss), map 
> it.
> +                 */
> +                if (eppnt->p_filesz < eppnt->p_memsz) {
> +                    zero_bss(vaddr_ef, vaddr_em, elf_prot);
> +                }
> +            } else if (eppnt->p_memsz != 0) {
> +                vaddr_len = TARGET_ELF_PAGELENGTH(eppnt->p_memsz + vaddr_po);
> +                error = target_mmap(vaddr_ps, vaddr_len, elf_prot,
> +                                    MAP_PRIVATE | MAP_FIXED | MAP_ANONYMOUS,
> +                                    -1, 0);
>  
> -            /* If the load segment requests extra zeros (e.g. bss), map it.  
> */
> -            if (vaddr_ef < vaddr_em) {
> -                zero_bss(vaddr_ef, vaddr_em, elf_prot);
> +                if (error == -1) {
> +                    goto exit_mmap;
> +                }
>              }
>  
>              /* Find the full program boundaries.  */
> 

Applied to my linux-user-for-6.0 branch.

Thanks,
Laurent