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SubjectProposal to fix pwrite with O_APPEND via pwritev2 flag
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.

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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