linux-kernel - Re: [PATCH] openat2: switch to __attribute__((packed)) for open

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Date:   Mon, 16 Dec 2019 07:55:39 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Florian Weimer <fw@...eb.enyo.de>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Shuah Khan <shuah@...nel.org>, dev@...ncontainers.org,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH] openat2: switch to __attribute__((packed)) for open_how

On 2019-12-15, Florian Weimer <fw@...eb.enyo.de> wrote:
> * Aleksa Sarai:
> 
> > diff --git a/tools/testing/selftests/openat2/helpers.h b/tools/testing/selftests/openat2/helpers.h
> > index 43ca5ceab6e3..eb1535c8fa2e 100644
> > --- a/tools/testing/selftests/openat2/helpers.h
> > +++ b/tools/testing/selftests/openat2/helpers.h
> > @@ -32,17 +32,16 @@
> >   * O_TMPFILE} are set.
> >   *
> >   * @flags: O_* flags.
> > - * @mode: O_CREAT/O_TMPFILE file mode.
> >   * @resolve: RESOLVE_* flags.
> > + * @mode: O_CREAT/O_TMPFILE file mode.
> >   */
> >  struct open_how {
> > -	__aligned_u64 flags;
> > +	__u64 flags;
> > +	__u64 resolve;
> >  	__u16 mode;
> > -	__u16 __padding[3]; /* must be zeroed */
> > -	__aligned_u64 resolve;
> > -};
> > +} __attribute__((packed));
> >  
> > -#define OPEN_HOW_SIZE_VER0	24 /* sizeof first published struct */
> > +#define OPEN_HOW_SIZE_VER0	18 /* sizeof first published struct */
> >  #define OPEN_HOW_SIZE_LATEST	OPEN_HOW_SIZE_VER0
> 
> A userspace ABI that depends on GCC extensions probably isn't a good
> idea.  Even with GCC, it will not work well with some future
> extensions because it pretty much rules out having arrays or other
> members that are access through pointers.  Current GCC does not carry
> over the packed-ness of the struct to addresses of its members.

Right, those are also good points.

Okay, I'm going to send a separate patch which changes the return value
for invalid __padding to -E2BIG, and moves the padding to the end of the
struct (along with open_how.mode). That should fix all of the warts I
raised, without running into the numerous problems with
__attribute__((packed)) of which I am now aware.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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