linux-kernel - Re: [PATCH v2] binfmt_misc: pass info about P flag by AT

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Message-ID: <7af5c24d-dd24-b728-92cf-a5a759787590@vivier.eu>
Date:   Thu, 16 Jan 2020 08:54:06 +0100
From:   Laurent Vivier <laurent@...ier.eu>
To:     YunQiang Su <syq@...ian.org>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     YunQiang Su <ysu@...ecomp.com>
Subject: Re: [PATCH v2] binfmt_misc: pass info about P flag by AT_FLAGS

Le 16/01/2020 à 03:20, YunQiang Su a écrit :
> From: YunQiang Su <ysu@...ecomp.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@...ecomp.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?

Al?

Thanks,
Laurent