From mboxrd@z Thu Jan 1 00:00:00 1970 From: Aleksa Sarai Subject: Re: [PATCH v9 08/10] open: openat2(2) syscall Date: Fri, 19 Jul 2019 01:21:23 +1000 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> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="x55cx7xfpotu25go" Return-path: Content-Disposition: inline In-Reply-To: <845e4364-685f-343b-46fb-c418766dce3e@rasmusvillemoes.dk> Sender: linux-kernel-owner@vger.kernel.org To: Rasmus Villemoes Cc: Al Viro , Jeff Layton , "J. Bruce Fields" , Arnd Bergmann , David Howells , Shuah Khan , Shuah Khan , Christian Brauner , Eric Biederman , Andy Lutomirski , Andrew Morton , Alexei Starovoitov , Kees Cook , Jann Horn , Tycho Andersen , David Drysdale , Chanho Min , Oleg Nesterov , Aleksa Sarai , Linus Torvalds , containers@lists.linux-f List-Id: linux-api@vger.kernel.org --x55cx7xfpotu25go Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2019-07-18, Rasmus Villemoes 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 > > =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 --x55cx7xfpotu25go Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQSxZm6dtfE8gxLLfYqdlLljIbnQEgUCXTCOcAAKCRCdlLljIbnQ EllZAP4qSUDEVdU4aP8+s9uysbQoCi6l463vJM+jdHxpJ66OfQEAxlI5lXwcL6G0 jPCtI0Vs5LI5kpJuE2k98ol8BVMyZAg= =2lKS -----END PGP SIGNATURE----- --x55cx7xfpotu25go--