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 */
