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

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Message-ID: <878sg4y6f9.fsf@x220.int.ebiederm.org>
Date:   Tue, 30 Jun 2020 09:28:10 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
        Greg Kroah-Hartman <greg@...ah.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        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

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.

Eric