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 */
