linux-kernel - Re: [PATCH v2 10/15] exec: Remove do_execve

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Date:   Tue, 30 Jun 2020 09:55:36 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
        Greg Kroah-Hartman <greg@...ah.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>, bpf <bpf@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Gary Lin <GLin@...e.com>, Bruno Meneguele <bmeneg@...hat.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 10/15] exec: Remove do_execve_file

On Tue, Jun 30, 2020 at 09:28:10AM -0500, Eric W. Biederman wrote:
> Christoph Hellwig <hch@...radead.org> writes:
> 
> > On Tue, Jun 30, 2020 at 07:14:23AM -0500, Eric W. Biederman wrote:
> >> Christoph Hellwig <hch@...radead.org> writes:
> >> 
> >> > FYI, this clashes badly with my exec rework.  I'd suggest you
> >> > drop everything touching exec here for now, and I can then
> >> > add the final file based exec removal to the end of my series.
> >> 
> >> I have looked and I haven't even seen any exec work.  Where can it be
> >> found?
> >> 
> >> I have working and cleaning up exec for what 3 cycles now.  There is
> >> still quite a ways to go before it becomes possible to fix some of the
> >> deep problems in exec.  Removing all of these broken exec special cases
> >> is quite frankly the entire point of this patchset.
> >> 
> >> Sight unseen I suggest you send me your exec work and I can merge it
> >> into my branch if we are going to conflict badly.
> >
> > https://lore.kernel.org/linux-fsdevel/20200627072704.2447163-1-hch@lst.de/T/#t
> 
> 
> Looking at your final patch I do not like the construct.
> 
> static int __do_execveat(int fd, struct filename *filename,
>  		const char __user *const __user *argv,
>  		const char __user *const __user *envp,
> 		const char *const *kernel_argv,
> 		const char *const *kernel_envp,
>  		int flags, struct file *file);
> 
> 
> It results in a function that is full of:
> 	if (kernel_argv) {
>         	// For kernel_exeveat 
> 		...
> 	} else {
>         	// For ordinary exeveat
>         	
>         }
> 
> Which while understandable.  I do not think results in good long term
> maintainble code.
> 
> The current file paramter that I am getting rid of in my patchset is
> a stark example of that.  Because of all of the if's no one realized
> that the code had it's file reference counting wrong (amoung other
> bugs).
> 
> I think this is important to address as exec has already passed
> the point where people can fix all of the bugs in exec because
> the code is so hairy.
> 
> I think to be maintainable and clear the code exec code is going to
> need to look something like:
> 
> static int bprm_execveat(int fd, struct filename *filename,
> 			struct bprm *bprm, int flags);
> 
> int kernel_execve(const char *filename,
> 		  const char *const *argv, const char *const *envp, int flags)
> {
> 	bprm = kzalloc(sizeof(*pbrm), GFP_KERNEL);
>         bprm->argc = count_kernel_strings(argv);
>         bprm->envc = count_kernel_strings(envp);
>         prepare_arg_pages(bprm);
>         copy_strings_kernel(bprm->envc, envp, bprm);
>         copy_strings_kernel(bprm->argc, argc, bprm);
> 	ret = bprm_execveat(AT_FDCWD, filename, bprm);
>         free_bprm(bprm);
>         return ret;
> }
> 
> int do_exeveat(int fd, const char *filename,
> 		const char __user *const __user *argv,
>                 const char __user *const __user *envp, int flags)
> {
> 	bprm = kzalloc(sizeof(*pbrm), GFP_KERNEL);
>         bprm->argc = count_strings(argv);
>         bprm->envc = count_strings(envp);
>         prepare_arg_pages(bprm);
>         copy_strings(bprm->envc, envp, bprm);
>         copy_strings(bprm->argc, argc, bprm);
> 	ret = bprm_execveat(fd, filename, bprm);
>         free_bprm(bprm);
>         return ret;
> }
> 
> More work is required obviously to make the code above really work but
> when the dust clears a structure like that doesn't have funny edge cases
> that can hide bugs and make it tricky to change the code.

+1 to the approach.
I think Christoph's work need to be on top of Eric's.