linux-kernel - Re: [PATCH] Compiler Attributes: Introduce __access

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Date:   Sat, 24 Sep 2022 07:58:54 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Siddhesh Poyarekar <siddhesh@...plt.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] Compiler Attributes: Introduce __access_*() function
 attribute

On Sat, Sep 24, 2022 at 12:06:31PM +0200, Miguel Ojeda wrote:
> On Sat, Sep 24, 2022 at 1:54 AM Kees Cook <keescook@...omium.org> wrote:
> >
> > are specified. While it is legal to provide only the pointer argument
> > position and access type, design the kernel macros to require also the
> > bounds (element count) argument position: if a function has no bounds
> > argument, refactor the code to include one.
> 
> Should this bit be included in the comment of the attribute? (since it
> is specific to the kernel)

Sure; good idea!

> 
> > These can be used multiple times. For example:
> >
> > __access_wo(2, 3) __access_ro(4, 5)
> > int copy_something(struct context *ctx, u32 *dst, size_t dst_count,
> >                    u8 *src, int src_len);
> >
> > (And if "dst" will also be read, it could use __access_rw(2, 3) instead.)
> 
> Also maybe the example could be nice there too, since it uses the
> syntax for the kernel and you took the time to write it :)
> 
> By the way, shouldn't `src` typically be `const u8 *`? Given it is an
> example, I would qualify it.

Yeah, I will update this.

> 
> > +#if __has_attribute(__access__)
> > +#define __access_rw(ptr, count)        __attribute__((__access__(read_write, ptr, count)))
> > +#define __access_ro(ptr, count)        __attribute__((__access__(read_only,  ptr, count)))
> > +#define __access_wo(ptr, count)        __attribute__((__access__(write_only, ptr, count)))
> > +#else
> > +#define __access_rw(ptr, count)
> > +#define __access_ro(ptr, count)
> > +#define __access_wo(ptr, count)
> > +#endif
> 
> If you do a v2 for the above, please take the chance to indent with a
> single space after the `#` (like `# define`) for consistency.

Fixed! I will send a v2.

-- 
Kees Cook