On Fri, Sep 14, 2018 at 8:08 PM Prakash Sangappa <prakash.sangappa@xxxxxxxxxx> wrote: > On 9/14/18 5:49 AM, Jann Horn wrote: > > On Fri, Sep 14, 2018 at 8:21 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > >> On Fri 14-09-18 03:33:28, Jann Horn wrote: > >>> On Wed, Sep 12, 2018 at 10:43 PM prakash.sangappa > >>> <prakash.sangappa@xxxxxxxxxx> wrote: > >>>> On 05/09/2018 04:31 PM, Dave Hansen wrote: > >>>>> On 05/07/2018 06:16 PM, prakash.sangappa wrote: > >>>>>> It will be /proc/<pid>/numa_vamaps. Yes, the behavior will be > >>>>>> different with respect to seeking. Output will still be text and > >>>>>> the format will be same. > >>>>>> > >>>>>> I want to get feedback on this approach. > >>>>> I think it would be really great if you can write down a list of the > >>>>> things you actually want to accomplish. Dare I say: you need a > >>>>> requirements list. > >>>>> > >>>>> The numa_vamaps approach continues down the path of an ever-growing list > >>>>> of highly-specialized /proc/<pid> files. I don't think that is > >>>>> sustainable, even if it has been our trajectory for many years. > >>>>> > >>>>> Pagemap wasn't exactly a shining example of us getting new ABIs right, > >>>>> but it sounds like something along those is what we need. > >>>> Just sent out a V2 patch. This patch simplifies the file content. It > >>>> only provides VA range to numa node id information. > >>>> > >>>> The requirement is basically observability for performance analysis. > >>>> > >>>> - Need to be able to determine VA range to numa node id information. > >>>> Which also gives an idea of which range has memory allocated. > >>>> > >>>> - The proc file /proc/<pid>/numa_vamaps is in text so it is easy to > >>>> directly view. > >>>> > >>>> The V2 patch supports seeking to a particular process VA from where > >>>> the application could read the VA to numa node id information. > >>>> > >>>> Also added the 'PTRACE_MODE_READ_REALCREDS' check when opening the > >>>> file /proc file as was indicated by Michal Hacko > >>> procfs files should use PTRACE_MODE_*_FSCREDS, not PTRACE_MODE_*_REALCREDS. > >> Out of my curiosity, what is the semantic difference? At least > >> kernel_move_pages uses PTRACE_MODE_READ_REALCREDS. Is this a bug? > > No, that's fine. REALCREDS basically means "look at the caller's real > > UID for the access check", while FSCREDS means "look at the caller's > > filesystem UID". The ptrace access check has historically been using > > the real UID, which is sorta weird, but normally works fine. Given > > that this is documented, I didn't see any reason to change it for most > > things that do ptrace access checks, even if the EUID would IMO be > > more appropriate. But things that capture caller credentials at points > > like open() really shouldn't look at the real UID; instead, they > > should use the filesystem UID (which in practice is basically the same > > as the EUID). > > > > So in short, it depends on the interface you're coming through: Direct > > syscalls use REALCREDS, things that go through the VFS layer use > > FSCREDS. > > So in this case can the REALCREDS check be done in the read() system call > when reading the /proc file instead of the open call? No, REALCREDS shouldn't be used in open() and shouldn't be used in read(). FSCREDS can be used in open(); in theory, using ptrace_may_access() in any way in read() is currently unsafe, but in practice, it's used that way anyway. I have plans to clean that up eventually...
