On 1/28/20 10:19 AM, Jens Axboe wrote: > On 1/28/20 9:19 AM, Jens Axboe wrote: >> On 1/28/20 9:17 AM, Stefan Metzmacher wrote: >>> Am 28.01.20 um 17:10 schrieb Jens Axboe: >>>> On 1/28/20 3:18 AM, Stefan Metzmacher wrote: >>>>> Hi Jens, >>>>> >>>>> now that we have IORING_FEAT_CUR_PERSONALITY... >>>>> >>>>> How can we optimize the fileserver case now, in order to avoid the >>>>> overhead of always calling 5 syscalls before io_uring_enter()?: >>>>> >>>>> /* gain root again */ >>>>> setresuid(-1,0,-1); setresgid(-1,0,-1) >>>>> /* impersonate the user with groups */ >>>>> setgroups(num, grps); setresgid(-1,gid,-1); setresuid(-1,uid,-1); >>>>> /* trigger the operation */ >>>>> io_uring_enter(); >>>>> >>>>> I guess some kind of IORING_REGISTER_CREDS[_UPDATE] would be >>>>> good, together with a IOSQE_FIXED_CREDS in order to specify >>>>> credentials per operation. >>>>> >>>>> Or we make it much more generic and introduce a credsfd_create() >>>>> syscall in order to get an fd for a credential handle, maybe >>>>> together with another syscall to activate the credentials of >>>>> the current thread (or let a write to the fd trigger the activation >>>>> in order to avoid an additional syscall number). >>>>> >>>>> Having just an fd would allow IORING_REGISTER_CREDS[_UPDATE] >>>>> to be just an array of int values instead of a more complex >>>>> structure to define the credentials. >>>> >>>> I'd rather avoid having to add more infrastructure for this, even if >>>> credsfd_create() would be nifty. >>>> >>>> With that in mind, something like: >>>> >>>> - Application does IORING_REGISTER_CREDS, which returns some index >>>> >>>> - Add a IORING_OP_USE_CREDS opcode, which sets the creds associated >>>> with dependent commands >>>> - Actual request is linked to the IORING_OP_USE_CREDS command, any >>>> link off IORING_OP_USE_CREDS will use those credentials >>> >>> Using links for this sounds ok. >> >> Great! I'll try and hack this up and see how it turns out. >> >>>> - IORING_UNREGISTER_CREDS removes the registered creds >>>> >>>> Just throwing that out there, definitely willing to entertain other >>>> methods that make sense for this. Trying to avoid needing to put this >>>> information in the SQE itself, hence the idea to use a chain of links >>>> for it. >>>> >>>> The downside is that we'll need to maintain an array of key -> creds, >>>> but that's probably not a big deal. >>>> >>>> What do you think? >>> >>> So IORING_REGISTER_CREDS would be a simple operation that just takes a >>> snapshot of the current_cred() and returns an id that can be passed to >>> IORING_OP_USE_CREDS or IORING_UNREGISTER_CREDS? >> >> Right, you would not pass in any arguments, it'd have to be run from the >> personality you wish to register. It simply returns an integer, which is >> a key to use for IORING_OP_USE_CREDS, or at the end for >> IORING_UNREGISTER_CREDS when you no longer wish to use this personality. >> >>>> Ideally I'd like to get this done for 5.6 even if we >>>> are a bit late, so you'll have everything you need with that release. >>> >>> That would be great! >> >> Crossing fingers... > > OK, so here are two patches for testing: > > https://git.kernel.dk/cgit/linux-block/log/?h=for-5.6/io_uring-vfs-creds > > #1 adds support for registering the personality of the invoking task, > and #2 adds support for IORING_OP_USE_CREDS. Right now it's limited to > just having one link, it doesn't support a chain of them. > > I'll try and write a test case for this just to see if it actually works, > so far it's totally untested. > > Adding Pavel to the CC. Minor tweak to ensuring we do the right thing for async offload as well, and it tests fine for me. Test case is: - Run as root - Register personality for root - create root only file - check we can IORING_OP_OPENAT the file - switch to user id test - check we cannot IORING_OP_OPENAT the file - check that we can open the file with IORING_OP_USE_CREDS linked -- Jens Axboe
