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    SubjectRe: [PATCH v9 08/10] open: openat2(2) syscall
    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 path *path, int flags,
    > > }
    > > EXPORT_SYMBOL(open_with_fake_path);
    > >
    > > -static inline int build_open_flags(int flags, umode_t mode, struct open_flags *op)
    > > +static inline int build_open_flags(struct open_how how, struct open_flags *op)
    > > {
    >
    > 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.

    > >
    > > 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 @@
    > >
    > > #include <uapi/linux/fcntl.h>
    > >
    > > -/* 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)
    > > +
    >
    > 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).

    >
    > > +/**
    > > + * 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).
    >
    > should probably say "O_CREAT/O_TMPFILE file mode".

    :+1:

    > > + * @upgrade_mask: restrict how the O_PATH may be re-opened (ignored otherwise).
    > > + * @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;
    >
    > 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?

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