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Message-ID: <20230827194122.GA325446@ZenIV> Date: Sun, 27 Aug 2023 20:41:22 +0100 From: Al Viro <viro@...iv.linux.org.uk> To: Christoph Hellwig <hch@....de> Cc: Matthew Wilcox <willy@...radead.org>, Jens Axboe <axboe@...nel.dk>, Xiubo Li <xiubli@...hat.com>, Ilya Dryomov <idryomov@...il.com>, Christian Brauner <brauner@...nel.org>, Theodore Ts'o <tytso@....edu>, Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>, Miklos Szeredi <miklos@...redi.hu>, Andreas Gruenbacher <agruenba@...hat.com>, "Darrick J. Wong" <djwong@...nel.org>, Trond Myklebust <trond.myklebust@...merspace.com>, Anna Schumaker <anna@...nel.org>, Damien Le Moal <dlemoal@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, linux-block@...r.kernel.org, ceph-devel@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com, linux-xfs@...r.kernel.org, linux-nfs@...r.kernel.org, linux-mm@...ck.org, Hannes Reinecke <hare@...e.de> Subject: Re: [PATCH 03/12] filemap: update ki_pos in generic_perform_write On Thu, Jun 01, 2023 at 04:58:55PM +0200, Christoph Hellwig wrote: > All callers of generic_perform_write need to updated ki_pos, move it into > common code. > @@ -4034,7 +4037,6 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from) > endbyte = pos + status - 1; > err = filemap_write_and_wait_range(mapping, pos, endbyte); > if (err == 0) { > - iocb->ki_pos = endbyte + 1; > written += status; > invalidate_mapping_pages(mapping, > pos >> PAGE_SHIFT, > @@ -4047,8 +4049,6 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from) > } > } else { > written = generic_perform_write(iocb, from); > - if (likely(written > 0)) > - iocb->ki_pos += written; > } > out: > return written ? written : err; [another late reply, sorry] That part is somewhat fishy - there's a case where you return a positive value and advance ->ki_pos by more than that amount. I really wonder if all callers of ->write_iter() are OK with that. Consider e.g. this: ssize_t ksys_write(unsigned int fd, const char __user *buf, size_t count) { struct fd f = fdget_pos(fd); ssize_t ret = -EBADF; if (f.file) { loff_t pos, *ppos = file_ppos(f.file); if (ppos) { pos = *ppos; ppos = &pos; } ret = vfs_write(f.file, buf, count, ppos); if (ret >= 0 && ppos) f.file->f_pos = pos; fdput_pos(f); } return ret; } ssize_t vfs_write(struct file *file, const char __user *buf, size_t count, loff_t *pos) { ssize_t ret; if (!(file->f_mode & FMODE_WRITE)) return -EBADF; if (!(file->f_mode & FMODE_CAN_WRITE)) return -EINVAL; if (unlikely(!access_ok(buf, count))) return -EFAULT; ret = rw_verify_area(WRITE, file, pos, count); if (ret) return ret; if (count > MAX_RW_COUNT) count = MAX_RW_COUNT; file_start_write(file); if (file->f_op->write) ret = file->f_op->write(file, buf, count, pos); else if (file->f_op->write_iter) ret = new_sync_write(file, buf, count, pos); else ret = -EINVAL; if (ret > 0) { fsnotify_modify(file); add_wchar(current, ret); } inc_syscw(current); file_end_write(file); return ret; } static ssize_t new_sync_write(struct file *filp, const char __user *buf, size_t len, loff_t *ppos) { struct kiocb kiocb; struct iov_iter iter; ssize_t ret; init_sync_kiocb(&kiocb, filp); kiocb.ki_pos = (ppos ? *ppos : 0); iov_iter_ubuf(&iter, ITER_SOURCE, (void __user *)buf, len); ret = call_write_iter(filp, &kiocb, &iter); BUG_ON(ret == -EIOCBQUEUED); if (ret > 0 && ppos) *ppos = kiocb.ki_pos; return ret; } Suppose ->write_iter() ends up doing returning a positive value smaller than the increment of kiocb.ki_pos. What do we get? ret is positive, so kiocb.ki_pos gets copied into *ppos, which is ksys_write's pos and there we copy it into file->f_pos. Is it really OK to have write() return 4096 and advance the file position by 16K? AFAICS, userland wouldn't get any indication of something odd going on - just a short write to a regular file, with followup write of remaining 12K getting quietly written in the range 16K..28K. I don't remember what POSIX says about that, but it would qualify as nasty surprise for any userland program - sure, one can check fsync() results before closing the sucker and see if everything looks fine, but the way it's usually discussed could easily lead to assumption that (synchronous) O_DIRECT writes would not be affected by anything of that sort.
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