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SubjectRe: Proposal to fix pwrite with O_APPEND via pwritev2 flag
On Thu, Jan 23, 2020 at 07:02:43PM -0500, Rich Felker wrote:
> There's a longstanding unfixable (due to API stability) bug in the
> pwrite syscall:
>
> http://man7.org/linux/man-pages/man2/pwrite.2.html#BUGS
>
> whereby it wrongly honors O_APPEND if set, ignoring the caller-passed
> offset. Now that there's a pwritev2 syscall that takes a flags
> argument, it's possible to fix this without breaking stability by
> adding a new RWF_NOAPPEND flag, which callers that want the fixed
> behavior can then pass.
>
> I have a completely untested patch to add such a flag, but would like
> to get a feel for whether the concept is acceptable before putting
> time into testing it. If so, I'll submit this as a proper patch with
> detailed commit message etc. Draft is below.

I went ahead and tested this, and it works as intended, so I'll post a
proper patch with commit message.

Rich



> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index e0d909d35763..3a769a972f79 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3397,6 +3397,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
> {
> 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))
> @@ -3411,6 +3413,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
> ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
> if (flags & RWF_APPEND)
> ki->ki_flags |= IOCB_APPEND;
> + if (flags & RWF_NOAPPEND)
> + ki->ki_flags &= ~IOCB_APPEND;
> return 0;
> }
>
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 379a612f8f1d..591357d9b3c9 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -299,8 +299,11 @@ 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 */

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