Re: [PATCH] binfmt_elf: fix return value in case of interpreter load failure

Matthieu CASTET
2013-04-12T14:49:56Z

Hi Andrew,

thanks for your quick review.

Andrew Morton a écrit :
> On Thu, 11 Apr 2013 15:53:09 +0200 Matthieu CASTET <matthieu.castet@parrot.com> wrote:
>
>> The current code return the address instead of using PTR_ERR.
>
> I don't understand what you mean here - please describe this error in
> much more detail. Help people to identify the section of code which
> is being discussed.

I was speaking of

elf_entry = load_elf_interp(&loc->interp_elf_ex,
interpreter,
&interp_map_addr,
load_bias);
[...]
if (BAD_ADDR(elf_entry)) {
force_sig(SIGSEGV, current);
retval = IS_ERR((void *)elf_entry) ?
(int)elf_entry : -EINVAL;
goto out_free_dentry;
}

and was expecting we should use PTR_ERR when IS_ERR is true to match what is
done in [1].

But didn't saw that PTR_ERR((void *)elf_entry) and (int)elf_entry are equivalent.

>
>> Also the check is done after adding e_entry. This can cause weird behaviour
>> because -errno + loc->interp_elf_ex.e_entry can produce a valid address.
>
> Which check?

I am really confused here. Reading again the code this can't happen because if
load_elf_interp return -errno

We don't enter this condition
> if (!IS_ERR((void *)elf_entry)) {
> /*
> * load_elf_interp() returns relocation
> * adjustment
> */
> interp_load_addr = elf_entry;
> elf_entry += loc->interp_elf_ex.e_entry;
> }
we still have -errno here
> if (BAD_ADDR(elf_entry)) {
> force_sig(SIGSEGV, current);
> retval = IS_ERR((void *)elf_entry) ?
> (int)elf_entry : -EINVAL;
> goto out_free_dentry;
> }

Sorry for my mistake.

The only valid remaining part of my patch is to return SIGKILL when
load_elf_interp fail (IS_ERR((void *)elf_entry) is true) (for example load
address of linker is bad) instead of SIGSEGV. This will follow what is done when
loading binary.

But is it even worth doing?

>
>> Add a check to test load error before adding entry address. Also in this
>> case send SIGKILL instead of SIGSEGV to match what is done when loading binary.
>>
>> ...
>>
>> --- a/fs/binfmt_elf.c
>> +++ b/fs/binfmt_elf.c
>> @@ -900,18 +900,21 @@ static int load_elf_binary(struct linux_binprm *bprm)
>> interpreter,
>> &interp_map_addr,
>> load_bias);
>> - if (!IS_ERR((void *)elf_entry)) {
>> - /*
>> - * load_elf_interp() returns relocation
>> - * adjustment
>> - */
>> - interp_load_addr = elf_entry;
>> - elf_entry += loc->interp_elf_ex.e_entry;
>> + if (BAD_ADDR(elf_entry)) {
>> + force_sig(SIGKILL, current);
>> + retval = IS_ERR((void *)elf_entry) ?
>> + PTR_ERR((void *)elf_entry) : -EINVAL;
>
> Thats's a bit verbose - "PTR_ERR((void *)elf_entry)" is equivalent to
> "elf_entry". I suppose we can do it this way to document the intent or
> something.
Ok, I see.
Note that [1] use PTR_ERR but elf_map already return unsigned long like
load_elf_interp.

Matthieu

[1]
error = elf_map(bprm->file, load_bias + vaddr, elf_ppnt,
elf_prot, elf_flags, 0);
if (BAD_ADDR(error)) {
send_sig(SIGKILL, current, 0);
retval = IS_ERR((void *)error) ?
PTR_ERR((void*)error) : -EINVAL;
goto out_free_dentry;
}