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To:     YunQiang Su <syq@debian.org>, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org,
        James.Bottomley@hansenpartnership.com, viro@zeniv.linux.org.uk
Cc:     YunQiang Su <ysu@wavecomp.com>
References: <20200116083031.174367-1-syq@debian.org>
From:   Laurent Vivier <laurent@vivier.eu>
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Subject: Re: [PATCH v3] binfmt_misc: pass info about P flag by AT_FLAGS
Message-ID: <47d06a3c-eb5c-7824-b443-0f0e64fab38b@vivier.eu>
Date:   Thu, 16 Jan 2020 09:52:07 +0100
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Le 16/01/2020 à 09:30, 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 0st bits of unused AT_FLAGS.
> Then, in qemu, we can get this info by:
>    getauxval(AT_FLAGS) & (1<<0)
> 
> v2->v3:
>   define a new AT_FLAGS_PRESERVE_ARGV0 as (1<<0), so now we use 0st bit.
> 
> 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..c4efff74223f 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 |= AT_FLAGS_PRESERVE_ARGV0;
> +	}
> +	NEW_AUX_ENT(AT_FLAGS, flags);
>  	NEW_AUX_ENT(AT_ENTRY, e_entry);
>  	NEW_AUX_ENT(AT_UID, from_kuid_munged(cred->user_ns, cred->uid));
>  	NEW_AUX_ENT(AT_EUID, from_kuid_munged(cred->user_ns, cred->euid));
> diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
> index 240f66663543..c89a4630efad 100644
> --- a/fs/binfmt_elf_fdpic.c
> +++ b/fs/binfmt_elf_fdpic.c
> @@ -507,6 +507,7 @@ static int create_elf_fdpic_tables(struct linux_binprm *bprm,
>  	char __user *u_platform, *u_base_platform, *p;
>  	int loop;
>  	int nr;	/* reset for each csp adjustment */
> +	unsigned long flags = 0;
>  
>  #ifdef CONFIG_MMU
>  	/* In some cases (e.g. Hyper-Threading), we want to avoid L1 evictions
> @@ -647,7 +648,10 @@ static int create_elf_fdpic_tables(struct linux_binprm *bprm,
>  	NEW_AUX_ENT(AT_PHENT,	sizeof(struct elf_phdr));
>  	NEW_AUX_ENT(AT_PHNUM,	exec_params->hdr.e_phnum);
>  	NEW_AUX_ENT(AT_BASE,	interp_params->elfhdr_addr);
> -	NEW_AUX_ENT(AT_FLAGS,	0);
> +	if (bprm->interp_flags & BINPRM_FLAGS_PRESERVE_ARGV0) {
> +		flags |= AT_FLAGS_PRESERVE_ARGV0;
> +	}
> +	NEW_AUX_ENT(AT_FLAGS,	flags);
>  	NEW_AUX_ENT(AT_ENTRY,	exec_params->entry_addr);
>  	NEW_AUX_ENT(AT_UID,	(elf_addr_t) from_kuid_munged(cred->user_ns, cred->uid));
>  	NEW_AUX_ENT(AT_EUID,	(elf_addr_t) from_kuid_munged(cred->user_ns, cred->euid));
> diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
> index cdb45829354d..cb14e9bbf00f 100644
> --- a/fs/binfmt_misc.c
> +++ b/fs/binfmt_misc.c
> @@ -158,6 +158,8 @@ static int load_misc_binary(struct linux_binprm *bprm)
>  		retval = remove_arg_zero(bprm);
>  		if (retval)
>  			goto ret;
> +	} else {
> +		bprm->interp_flags |= AT_FLAGS_PRESERVE_ARGV0;

Keep BINPRM_FLAGS_PRESERVE_ARGV0 here.

>  	}
>  
>  	if (fmt->flags & MISC_FMT_OPEN_BINARY) {
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index b40fc633f3be..380a30a46db1 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -78,6 +78,10 @@ struct linux_binprm {
>  #define BINPRM_FLAGS_PATH_INACCESSIBLE_BIT 2
>  #define BINPRM_FLAGS_PATH_INACCESSIBLE (1 << BINPRM_FLAGS_PATH_INACCESSIBLE_BIT)
>  
> +/* if preserve the argv0 for the interpreter  */
> +#define AT_FLAGS_PRESERVE_ARGV0_BIT 0
> +#define AT_FLAGS_PRESERVE_ARGV0 (1 << AT_FLAGS_PRESERVE_ARGV0_BIT)
> +

I think it should be added in include/uapi/linux/binfmts.h to be shared
with user space (external API-

Where is defined BINPRM_FLAGS_PRESERVE_ARGV0 now? It should be here
(internal ABI)

Thanks,
Laurent