Re: [PATCH v4 1/2] qemu-binfmt-conf.sh: allow elf EI_ABIVERSION=1 for mips

[WANG Xuerui]

Hi Andreas,

On 3/24/22 07:05, Andreas K. Hüttel wrote:
> With the command line flag -mplt and a recent toolchain, ELF binaries
> generated by gcc can obtain EI_ABIVERSION=1, see below, which makes, e.g.,
> gcc three-stage bootstrap in a mips-unknown-linux-gnu qemu-user chroot
> fail since the binfmt-misc magic does not match anymore.
>
> qemu executes these binaries just fine, so relax the mask slightly.
>
> CHOST=mips-unknown-linux-gnu (and also mipsel-unknown-linux-gnu)
> CFLAGS="-O2 -march=mips32 -mabi=32 -mplt -pipe"
> gcc-11.2, binutils-2.37, glibc-2.34
>
> |  /*
> | - * ELF dump of './prev-gcc/build/gengenrtl'
> | - *     29608 (0x73A8) bytes
> | + * ELF dump of './gcc/build/gengenrtl'
> | + *     54532 (0xD504) bytes
> |   */
> |
> |  Elf32_Dyn dumpedelf_dyn_0[];
> |  struct {
> |         Elf32_Ehdr ehdr;
> |         Elf32_Phdr phdrs[12];
> | -       Elf32_Shdr shdrs[33];
> | +       Elf32_Shdr shdrs[44];
> |         Elf32_Dyn *dyns;
> |  } dumpedelf_0 = {
> |
> |  .ehdr = {
> |         .e_ident = { /* (EI_NIDENT bytes) */
> |                 /* [0] EI_MAG:        */ 0x7F,'E','L','F',
> |                 /* [4] EI_CLASS:      */ 1 , /* (ELFCLASS32) */
> |                 /* [5] EI_DATA:       */ 2 , /* (ELFDATA2MSB) */
> |                 /* [6] EI_VERSION:    */ 1 , /* (EV_CURRENT) */
> |                 /* [7] EI_OSABI:      */ 0 , /* (ELFOSABI_NONE) */
> | -               /* [8] EI_ABIVERSION: */ 0 ,
> | +               /* [8] EI_ABIVERSION: */ 1 ,

It seems we could have more values than 0 or 1 here, according to 
binutils sources [1], so we might as well relax the mask for this byte...

> |                 /* [9-15] EI_PAD:     */ 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
> |         },
> |         .e_type      = 2          , /* (ET_EXEC) */
> |         .e_machine   = 8          , /* (EM_MIPS) */
> |         .e_version   = 1          , /* (EV_CURRENT) */
> | (...)
>
> Signed-off-by: Andreas K. Hüttel <dilfridge@gentoo.org>
> ---
>   scripts/qemu-binfmt-conf.sh | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/scripts/qemu-binfmt-conf.sh b/scripts/qemu-binfmt-conf.sh
> index e9bfeb94d3..fc2f856800 100755
> --- a/scripts/qemu-binfmt-conf.sh
> +++ b/scripts/qemu-binfmt-conf.sh
> @@ -61,11 +61,11 @@ m68k_family=m68k
>   # FIXME: We could use the other endianness on a MIPS host.
>   
>   mips_magic='\x7fELF\x01\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x08'
> -mips_mask='\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff'
> +mips_mask='\xff\xff\xff\xff\xff\xff\xff\x00\xfe\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff'

... to just \x00, to allow any future MIPS libc ABI bumps. Libc ABI 
should not interfere with instruction semantics and emulation, so we're 
safe.

>   mips_family=mips
>   
>   mipsel_magic='\x7fELF\x01\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x08\x00'
> -mipsel_mask='\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff'
> +mipsel_mask='\xff\xff\xff\xff\xff\xff\xff\x00\xfe\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff'
>   mipsel_family=mips
>   
>   mipsn32_magic='\x7fELF\x01\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x08'



[1]: 
https://github.com/bminor/binutils-gdb/blob/binutils-2_38/bfd/elfxx-mips.c#L16691-L16739

--
WANG Xuerui
xen0n@gentoo.org
Gentoo Linux developer
PGP: 7C52 19E3 26A0 7311 3EA3 8806 C01F 7214 BC93 1414