On 2019/08/25 18:59, Ingo Molnar wrote: >> @@ -132,8 +132,10 @@ static ssize_t read_mem(struct file *file, char __user >> *buf, >> #endif >> >> bounce = kmalloc(PAGE_SIZE, GFP_KERNEL); >> - if (!bounce) >> - return -ENOMEM; >> + if (!bounce) { >> + err = -ENOMEM; >> + goto failed; >> + } > > Yeah, so while I agree with the more consistent handling of partial > reads, I'd suggest the following changes: > > - Please don't use this 4-line error handling variant, use the old short > 2-line pattern instead. There's no real reason to keep 'err' as a > flag, the 'failed' branch will know that 'err' is the error return if > there's been no progress.
The caller might guarantee count > 0, but for robustness, I decided to choose 4-line error handling here because I merged the normal and the failure path control flow; read will remain 0 if count == 0, and thus err should remain 0. > > - We should probably separate out a third 'fatal error' variant: for > example if copying to user-space generates a page fault, then we > clearly should not pretend that all is fine and return a short read > even if we made some progress, a -EFAULT is more informative, as we > might have corrupted (overran) some user buffer on the failed copy as > well, and ran off the end into the first unmapped user area. Is it possible that copy_from_user() corrupts user buffer in a way that userspace cannot retry when kernel responded with "there was a short write"? It seems that these functions are difficult to return appropriate errors... > > - As for the patch series maybe it might make sense to separate the > fixes from the semantic changes, in case there's any breakage. I.e. > first fix the bug minimally, then add the other changes in a separate > commit. If any of them causes problems with applications we'll have a > more precise bisection result. Yes. I think for now we should just make these functions killable. Then, we can try changing return code for partial read/write. If no breakage report, we can make these functions interruptible. > > - Likewise, the changing of the write side interruptability of /dev/mem > should probably be a separate patch as well. > > I can factor out such a series if you don't have the time, but feel free > to do it yourself, this is your bug report and your patch. :) You can do it. It's a syzbot's bug report. I just forwarded it. ;-)