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From:   Jann Horn <jannh@google.com>
Date:   Mon, 31 Aug 2020 17:46:19 +0200
Message-ID: <CAG48ez39WNuoxYO=RaW5OeVGSOy=uEAZ+xW_++TP7yjkUKGqkg@mail.gmail.com>
Subject: Re: [PATCH v2] vfs: add RWF_NOAPPEND flag for pwritev2
To:     Rich Felker <dalias@libc.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>
Cc:     linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Pavel Begunkov <asml.silence@gmail.com>
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On Mon, Aug 31, 2020 at 5:32 PM Rich Felker <dalias@libc.org> wrote:
> The pwrite function, originally defined by POSIX (thus the "p"), is
> defined to ignore O_APPEND and write at the offset passed as its
> argument. However, historically Linux honored O_APPEND if set and
> ignored the offset. This cannot be changed due to stability policy,
> but is documented in the man page as a bug.
>
> Now that there's a pwritev2 syscall providing a superset of the pwrite
> functionality that has a flags argument, the conforming behavior can
> be offered to userspace via a new flag. Since pwritev2 checks flag
> validity (in kiocb_set_rw_flags) and reports unknown ones with
> EOPNOTSUPP, callers will not get wrong behavior on old kernels that
> don't support the new flag; the error is reported and the caller can
> decide how to handle it.
>
> Signed-off-by: Rich Felker <dalias@libc.org>

Reviewed-by: Jann Horn <jannh@google.com>

Note that if this lands, Michael Kerrisk will probably be happy if you
send a corresponding patch for the manpage man2/readv.2.

Btw, I'm not really sure whose tree this should go through - VFS is
normally Al Viro's turf, but it looks like the most recent
modifications to this function have gone through Jens Axboe's tree?

> ---
>
> Changes in v2: I've added a check to ensure that RWF_NOAPPEND does not
> override O_APPEND for S_APPEND (chattr +a) inodes, and fixed conflicts
> with 1752f0adea98ef85, which optimized kiocb_set_rw_flags to work with
> a local copy of flags. Unfortunately the same optimization does not
> work for RWF_NOAPPEND since it needs to remove flags from the original
> set at function entry.
>
> If desired, I could further change this so that kiocb_flags is
> initialized to ki->ki_flags, with assignment-back in place of |= at
> the end of the function. This would allow the same local variable
> pattern in the RWF_NOAPPEND code path, which might be more elegant,
> but I'm not sure if the emitted code would improve or get worse.
>
>
>  include/linux/fs.h      | 7 +++++++
>  include/uapi/linux/fs.h | 5 ++++-
>  2 files changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 7519ae003a08..924e17ac8e7e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3321,6 +3321,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
>                 return 0;
>         if (unlikely(flags & ~RWF_SUPPORTED))
>                 return -EOPNOTSUPP;
> +       if (unlikely((flags & RWF_APPEND) && (flags & RWF_NOAPPEND)))
> +               return -EINVAL;
>
>         if (flags & RWF_NOWAIT) {
>                 if (!(ki->ki_filp->f_mode & FMODE_NOWAIT))
> @@ -3335,6 +3337,11 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
>                 kiocb_flags |= (IOCB_DSYNC | IOCB_SYNC);
>         if (flags & RWF_APPEND)
>                 kiocb_flags |= IOCB_APPEND;
> +       if ((flags & RWF_NOAPPEND) && (ki->ki_flags & IOCB_APPEND)) {
> +               if (IS_APPEND(file_inode(ki->ki_filp)))
> +                       return -EPERM;
> +               ki->ki_flags &= ~IOCB_APPEND;
> +       }
>
>         ki->ki_flags |= kiocb_flags;
>         return 0;
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index f44eb0a04afd..d5e54e0742cf 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -300,8 +300,11 @@ typedef int __bitwise __kernel_rwf_t;
>  /* per-IO O_APPEND */
>  #define RWF_APPEND     ((__force __kernel_rwf_t)0x00000010)
>
> +/* per-IO negation of O_APPEND */
> +#define RWF_NOAPPEND   ((__force __kernel_rwf_t)0x00000020)
> +
>  /* mask of flags supported by the kernel */
>  #define RWF_SUPPORTED  (RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
> -                        RWF_APPEND)
> +                        RWF_APPEND | RWF_NOAPPEND)
>
>  #endif /* _UAPI_LINUX_FS_H */
> --
> 2.21.0
>