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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>
Link: https://lore.kernel.org/r/20200831153207.GO3265@brightrain.aerifal.cx
Reviewed-by: Jann Horn <jannh@google.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Rich Felker 2020-08-31 11:32:08 -04:00 committed by Christian Brauner
parent 9e3f1c5936
commit 73fa7547c7
2 changed files with 12 additions and 1 deletions
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8
include/linux/fs.h
View File

@ -3335,6 +3335,8 @@ 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))
@ -3345,6 +3347,12 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
if (flags & RWF_SYNC)
kiocb_flags |= IOCB_DSYNC;
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;
}

5
include/uapi/linux/fs.h
View File

@ -301,9 +301,12 @@ 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)
/* Pagemap ioctl */
#define PAGEMAP_SCAN _IOWR('f', 16, struct pm_scan_arg)