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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>
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2 changed files with 12 additions and 1 deletions
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@ -3335,6 +3335,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
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return 0;
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return 0;
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if (unlikely(flags & ~RWF_SUPPORTED))
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if (unlikely(flags & ~RWF_SUPPORTED))
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return -EOPNOTSUPP;
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return -EOPNOTSUPP;
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if (unlikely((flags & RWF_APPEND) && (flags & RWF_NOAPPEND)))
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return -EINVAL;
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if (flags & RWF_NOWAIT) {
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if (flags & RWF_NOWAIT) {
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if (!(ki->ki_filp->f_mode & FMODE_NOWAIT))
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if (!(ki->ki_filp->f_mode & FMODE_NOWAIT))
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@ -3345,6 +3347,12 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
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if (flags & RWF_SYNC)
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if (flags & RWF_SYNC)
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kiocb_flags |= IOCB_DSYNC;
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kiocb_flags |= IOCB_DSYNC;
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if ((flags & RWF_NOAPPEND) && (ki->ki_flags & IOCB_APPEND)) {
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if (IS_APPEND(file_inode(ki->ki_filp)))
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return -EPERM;
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ki->ki_flags &= ~IOCB_APPEND;
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}
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ki->ki_flags |= kiocb_flags;
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ki->ki_flags |= kiocb_flags;
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return 0;
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return 0;
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}
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}
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@ -301,9 +301,12 @@ typedef int __bitwise __kernel_rwf_t;
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/* per-IO O_APPEND */
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/* per-IO O_APPEND */
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#define RWF_APPEND ((__force __kernel_rwf_t)0x00000010)
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#define RWF_APPEND ((__force __kernel_rwf_t)0x00000010)
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/* per-IO negation of O_APPEND */
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#define RWF_NOAPPEND ((__force __kernel_rwf_t)0x00000020)
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/* mask of flags supported by the kernel */
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/* mask of flags supported by the kernel */
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#define RWF_SUPPORTED (RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
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#define RWF_SUPPORTED (RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
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RWF_APPEND)
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RWF_APPEND | RWF_NOAPPEND)
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/* Pagemap ioctl */
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/* Pagemap ioctl */
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#define PAGEMAP_SCAN _IOWR('f', 16, struct pm_scan_arg)
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#define PAGEMAP_SCAN _IOWR('f', 16, struct pm_scan_arg)
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