On Friday, February 12th, 2021 at 1:57 PM, Emil Velikov <emil.l.velikov@xxxxxxxxx> wrote: > On Fri, 5 Feb 2021 at 22:01, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote: > > > > Userspace has discovered the functionality offered by SYS_kcmp and has > > started to depend upon it. In particular, Mesa uses SYS_kcmp for > > os_same_file_description() in order to identify when two fd (e.g. device > > or dmabuf) > > As you rightfully point out, SYS_kcmp is a bit of a two edged sword. > While you mention the CONFIG issue, there is also a portability aspect > (mesa runs on more than just linux) and as well as sandbox filtering > of the extra syscall. > > Last time I looked, the latter was still an issue and mesa was using > SYS_kcmp to compare device node fds. > A far shorter and more portable solution is possible, so let me > prepare a Mesa patch. Comparing two DMA-BUFs can be done with their inode number, I think. Comparing two device FDs is more subtle, because of GEM handle ref'counting. You sometimes really want to check whether two FDs are backed by the same file *description*. See [1] for details. [1]: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/110
