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Subject[RESEND PATCH] vfs: add RWF_NOAPPEND flag for pwritev2
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>
---
include/linux/fs.h | 4 ++++
include/uapi/linux/fs.h | 5 ++++-
2 files changed, 8 insertions(+), 1 deletion(-)

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 */
--
2.21.0
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