linux-kernel - Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

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Message-ID: <CAGudoHGEOGJtcXFL2LVphNMpe38h3u4XT=PJZbc84Gka_h+tCg@mail.gmail.com>
Date:   Fri, 3 Mar 2023 22:09:43 +0100
From:   Mateusz Guzik <mjguzik@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Christian Brauner <brauner@...nel.org>, serge@...lyn.com,
        paul@...l-moore.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

On 3/3/23, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Fri, Mar 3, 2023 at 12:39 PM Mateusz Guzik <mjguzik@...il.com> wrote:
>>
>> I think there is a systemic problem which comes with the kzalloc API
>
> Well, it's not necessarily the API that is bad, but the implementation.
>
> We could easily make kzalloc() with a constant size just expand to
> kmalloc+memset, and get the behavior you want.
>
> We already do magical things for "find the right slab bucket" part of
> kmalloc too for constant sizes. It's changed over the years, but that
> policy goes back a long long time. See
>
>
> https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/?id=95203fe78007f9ab3aebb96606473ae18c00a5a8
>
> from the BK history tree.
>
> Exactly because some things are worth optimizing for when the size is
> known at compile time.
>
> Maybe just extending kzalloc() similarly? Trivial and entirely untested
> patch:
>
>    --- a/include/linux/slab.h
>    +++ b/include/linux/slab.h
>    @@ -717,6 +717,12 @@ static inline void *kmem_cache_zalloc(struct
> kmem_cache *k, gfp_t flags)
>      */
>     static inline __alloc_size(1) void *kzalloc(size_t size, gfp_t flags)
>     {
>    +    if (__builtin_constant_p(size)) {
>    +            void *ret = kmalloc(size, flags);
>    +            if (ret)
>    +                    memset(ret, 0, size);
>    +            return ret;
>    +    }
>         return kmalloc(size, flags | __GFP_ZERO);
>     }
>
> This may well be part of what has changed over the years. People have
> done a *lot* of pseudo-automated "kmalloc+memset -> kzalloc" code
> simplification. And in the process we've lost a lot of good
> optimizations.

I was about to write that kzalloc can use automagic treatment. I made
a change of similar sort years back in FreeBSD
https://cgit.freebsd.org/src/commit/?id=34c538c3560591a3856e85988b0b5eefdde53b0c

The crux of the comment though was not about kzalloc (another
brainfart, apologies), but kmem_cache_zalloc -- that one is kind of
screwed as is.

Perhaps it would be unscrewed if calls could be converted to something
in the lines of kmem_cache_zalloc_ptr(cachep, flags, returnobj);

so this from __alloc_file:
        struct file *f;
	int error;

	f = kmem_cache_zalloc(filp_cachep, GFP_KERNEL);
        if (unlikely(!f))
		return ERR_PTR(-ENOMEM);

could be:
	if (unlikely(!kmem_cache_zalloc_ptr(filp_cachep, GFP_KERNEL, f))
		return ERR_PTR(-ENOMEM);

... where the macro rolls with similar treatment to the one you pasted
for kzalloc. and assigns to f.

if this sounds acceptable coccinelle could be used to do a sweep

I don't have a good name for it.

-- 
Mateusz Guzik <mjguzik gmail.com>