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Date:   Fri, 19 Jul 2019 01:21:23 +1000
From:   Aleksa Sarai <cyphar@cyphar.com>
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Jeff Layton <jlayton@kernel.org>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Arnd Bergmann <arnd@arndb.de>,
        David Howells <dhowells@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Christian Brauner <christian@brauner.io>,
        Eric Biederman <ebiederm@xmission.com>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>, Tycho Andersen <tycho@tycho.ws>,
        David Drysdale <drysdale@google.com>,
        Chanho Min <chanho.min@lge.com>,
        Oleg Nesterov <oleg@redhat.com>, Aleksa Sarai <asarai@suse.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        containers@lists.linux-foundation.org, linux-alpha@vger.kernel.org,
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        linux-xtensa@linux-xtensa.org, sparclinux@vger.kernel.org
Subject: Re: [PATCH v9 08/10] open: openat2(2) syscall
Message-ID: <20190718152123.m33t44dapy6y4nwy@yavin>
References: <20190706145737.5299-1-cyphar@cyphar.com>
 <20190706145737.5299-9-cyphar@cyphar.com>
 <845e4364-685f-343b-46fb-c418766dce3e@rasmusvillemoes.dk>
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On 2019-07-18, Rasmus Villemoes <linux@rasmusvillemoes.dk> wrote:
> On 06/07/2019 16.57, Aleksa Sarai wrote:
> > --- a/fs/open.c
> > +++ b/fs/open.c
> > @@ -928,24 +928,32 @@ struct file *open_with_fake_path(const struct pat=
h *path, int flags,
> >  }
> >  EXPORT_SYMBOL(open_with_fake_path);
> > =20
> > -static inline int build_open_flags(int flags, umode_t mode, struct ope=
n_flags *op)
> > +static inline int build_open_flags(struct open_how how, struct open_fl=
ags *op)
> >  {
>=20
> How does passing such a huge struct by value affect code generation?
> Does gcc actually inline the function (and does it even inline the old
> one given that it's already non-trivial and has more than one caller).

I'm not sure, but I'll just do what you suggested with passing a const
reference and just copying the few fields that actually are touched by
this function.

> > =20
> > diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
> > index 2868ae6c8fc1..e59917292213 100644
> > --- a/include/linux/fcntl.h
> > +++ b/include/linux/fcntl.h
> > @@ -4,13 +4,26 @@
> > =20
> >  #include <uapi/linux/fcntl.h>
> > =20
> > -/* list of all valid flags for the open/openat flags argument: */
> > +/* Should open_how.mode be set for older syscalls wrappers? */
> > +#define OPENHOW_MODE(flags, mode) \
> > +	(((flags) | (O_CREAT | __O_TMPFILE)) ? (mode) : 0)
> > +
>=20
> Typo: (((flags) & (O_CREAT | __O_TMPFILE)) ? (mode) : 0)

Yup, thanks. I'm not sure why my tests passed on v9 with this bug (they
didn't pass in my v10-draft until I fixed this bug earlier today).

>=20
> > +/**
> > + * Arguments for how openat2(2) should open the target path. If @extra=
 is zero,
> > + * then openat2(2) is identical to openat(2).
> > + *
> > + * @flags: O_* flags (unknown flags ignored).
> > + * @mode: O_CREAT file mode (ignored otherwise).
>=20
> should probably say "O_CREAT/O_TMPFILE file mode".

:+1:

> > + * @upgrade_mask: restrict how the O_PATH may be re-opened (ignored ot=
herwise).
> > + * @resolve: RESOLVE_* flags (-EINVAL on unknown flags).
> > + * @reserved: reserved for future extensions, must be zeroed.
> > + */
> > +struct open_how {
> > +	__u32 flags;
> > +	union {
> > +		__u16 mode;
> > +		__u16 upgrade_mask;
> > +	};
> > +	__u16 resolve;
>=20
> So mode and upgrade_mask are naturally u16 aka mode_t. And yes, they
> probably never need to be used together, so the union works. That then
> makes the next member 2-byte aligned, so using a u16 for the resolve
> flags brings us to an 8-byte boundary, and 11 unused flag bits should be
> enough for a while. But it seems a bit artificial to cram all this
> together and then add 56 bytes of reserved space.

I will happily admit that padding to 64 bytes is probably _very_ extreme
(I picked it purely because it's the size of a cache-line so anything
bigger makes even less sense). I was hoping someone would suggest a
better size once I posted the patchset, since I couldn't think of a good
answer myself.

Do you have any suggestions for a better layout or padding size?

--=20
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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