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Subject: Re: [PATCH v2] binfmt_misc: pass info about P flag by AT_FLAGS
To:     YunQiang Su <syq@debian.org>
Cc:     linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        YunQiang Su <ysu@wavecomp.com>
References: <20200116022049.164659-1-syq@debian.org>
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 <CAKcpw6VEVaeBA9y1UC+vMzM=u9q4mc4Mr3FWewxDFkFBBtV1xA@mail.gmail.com>
From:   Laurent Vivier <laurent@vivier.eu>
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Message-ID: <a699c524-f7e6-c517-60c7-20cf182c286c@vivier.eu>
Date:   Thu, 16 Jan 2020 09:18:59 +0100
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Le 16/01/2020 à 09:17, YunQiang Su a écrit :
> Laurent Vivier <laurent@vivier.eu> 于2020年1月16日周四 下午4:07写道：
>>
>> Le 16/01/2020 à 03:20, YunQiang Su a écrit :
>>> From: YunQiang Su <ysu@wavecomp.com>
>>>
>>> Currently program invoked by binfmt_misc cannot be aware about whether
>>> P flag, aka preserve path is enabled.
>>>
>>> Some applications like qemu need to know since it has 2 use case:
>>>   1. call by hand, like: qemu-mipsel-static test.app OPTION
>>>      so, qemu have to assume that P option is not enabled.
>>>   2. call by binfmt_misc. If qemu cannot know about whether P flag is
>>>      enabled, distribution's have to set qemu without P flag, and
>>>      binfmt_misc call qemu like:
>>>        qemu-mipsel-static /absolute/path/to/test.app OPTION
>>>      even test.app is not called by absoulute path, like
>>>        ./relative/path/to/test.app
>>>
>>> This patch passes this information by the 3rd bits of unused AT_FLAGS.
>>> Then, in qemu, we can get this info by:
>>>    getauxval(AT_FLAGS) & (1<<3)
>>>
>>> v1->v2:
>>>   not enable kdebug
>>>
>>> See: https://bugs.launchpad.net/qemu/+bug/1818483
>>> Signed-off-by: YunQiang Su <ysu@wavecomp.com>
>>> ---
>>>  fs/binfmt_elf.c         | 6 +++++-
>>>  fs/binfmt_elf_fdpic.c   | 6 +++++-
>>>  fs/binfmt_misc.c        | 2 ++
>>>  include/linux/binfmts.h | 4 ++++
>>>  4 files changed, 16 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
>>> index f4713ea76e82..d33ee07d7f57 100644
>>> --- a/fs/binfmt_elf.c
>>> +++ b/fs/binfmt_elf.c
>>> @@ -178,6 +178,7 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
>>>       unsigned char k_rand_bytes[16];
>>>       int items;
>>>       elf_addr_t *elf_info;
>>> +     elf_addr_t flags = 0;
>>>       int ei_index;
>>>       const struct cred *cred = current_cred();
>>>       struct vm_area_struct *vma;
>>> @@ -252,7 +253,10 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
>>>       NEW_AUX_ENT(AT_PHENT, sizeof(struct elf_phdr));
>>>       NEW_AUX_ENT(AT_PHNUM, exec->e_phnum);
>>>       NEW_AUX_ENT(AT_BASE, interp_load_addr);
>>> -     NEW_AUX_ENT(AT_FLAGS, 0);
>>> +     if (bprm->interp_flags & BINPRM_FLAGS_PRESERVE_ARGV0) {
>>> +             flags |= BINPRM_FLAGS_PRESERVE_ARGV0;
>>> +     }
>>
>> Perhaps we also need a different flag in AT_FLAG than in interp_flag as
>> BINPRM_FLAGS_PRESERVE_ARGV0 is also part of the internal ABI?
> 
> yep. It may be really a problem.
> So, should we define a set of new macros for AT_FLAGS?

Yes, I think.

Thanks,
Laurent