[CC += Andreas, Linus, Roland, Markus; fixed Oleg] On 12/15/20 7:34 PM, Alejandro Colomar (man-pages) wrote: > Hi Ted, > > On 12/15/20 7:31 PM, Ted Estes wrote: >> Per my research on the topic, the error is in the manual page. The >> behavior of ptrace(2) was intentionally changed to prohibit attaching to >> a thread in the same group. Apparently, there were a number of >> ill-behaved edge cases. >> >> I found this email thread on the subject: >> https://lkml.org/lkml/2006/8/31/241 Okay, after reading the LKML thread, the old behavior was removed because it was very buggy. We have two options now: 1) Remove that paragraph, as if that behavior had never existed. If we do this, not much is lost: Only _very_ old kernels had that behavior, and it's not even advisable to make use of it on those, AFAICS. 2) Add a note to that paragraph, saying that since kernel 2.X.Y? the calling thread and the target thread can't be in the same group. Cons: That info is unlikely to be useful, and will only add a few more lines to a page that is already very long. 3) Suggestions? I prefer option 1. I'll add a larger screenshot of the manual page below, so that readers don't need to read 'man 2 ptrace': [[ ... The algorithm employed for ptrace access mode checking deter‐ mines whether the calling process is allowed to perform the corresponding action on the target process. (In the case of opening /proc/[pid] files, the "calling process" is the one opening the file, and the process with the corresponding PID is the "target process".) The algorithm is as follows: 1. If the calling thread and the target thread are in the same thread group, access is always allowed. 2. If the access mode specifies PTRACE_MODE_FSCREDS, then, for the check in the next step, employ the caller's filesystem UID and GID. (As noted in credentials(7), the filesystem UID and GID almost always have the same values as the corre‐ sponding effective IDs.) Otherwise, the access mode specifies PTRACE_MODE_REALCREDS, so use the caller's real UID and GID for the checks in the next step. (Most APIs that check the caller's UID and GID use the effective IDs. For historical reasons, the PTRACE_MODE_REALCREDS check uses the real IDs instead.) ... ]] Any thoughts before I write the patch? Thanks, Alex > > Thank you for all the details and links! > I'll fix the page. > > Thanks, > > Alex > >> >> Thank you. >> --Ted Estes >> >> On 12/15/2020 11:01 AM, Alejandro Colomar (man-pages) wrote: >>> Hi, >>> >>> There's a bug report: https://bugzilla.kernel.org/show_bug.cgi?id=210655 >>> >>> [[ >>> Under "Ptrace access mode checking", the documentation states: >>> "1. If the calling thread and the target thread are in the same thread >>> group, access is always allowed." >>> >>> This is incorrect. A thread may never attach to another in the same >>> group. >>> >>> Reference, ptrace_attach() >>> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/kernel/ptrace.c?h=v5.9.14#n380 >>> >>> ]] >>> >>> I just wanted to make sure that it is a bug in the manual page, and not >>> in the implementation. >>> >>> >>> Thanks, >>> >>> Alex >>> >> >
