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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <20200513141147.GD2719@gaia> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 5/13/20 11:11 AM, Catalin Marinas wrote: > On Wed, May 13, 2020 at 09:52:52AM -0300, Luis Machado wrote: >> On 5/13/20 7:48 AM, Catalin Marinas wrote: >>> On Tue, May 12, 2020 at 04:05:15PM -0300, Luis Machado wrote: >>>> On 4/21/20 11:25 AM, Catalin Marinas wrote: >>>>> Add support for bulk setting/getting of the MTE tags in a tracee's >>>>> address space at 'addr' in the ptrace() syscall prototype. 'data' points >>>>> to a struct iovec in the tracer's address space with iov_base >>>>> representing the address of a tracer's buffer of length iov_len. The >>>>> tags to be copied to/from the tracer's buffer are stored as one tag per >>>>> byte. >>>>> >>>>> On successfully copying at least one tag, ptrace() returns 0 and updates >>>>> the tracer's iov_len with the number of tags copied. In case of error, >>>>> either -EIO or -EFAULT is returned, trying to follow the ptrace() man >>>>> page. >>>>> >>>>> Note that the tag copying functions are not performance critical, >>>>> therefore they lack optimisations found in typical memory copy routines. >>>>> >>>>> Signed-off-by: Catalin Marinas >>>>> Cc: Will Deacon >>>>> Cc: Alan Hayward >>>>> Cc: Luis Machado >>>>> Cc: Omair Javaid >>>> >>>> I started working on MTE support for GDB and I'm wondering if we've already >>>> defined a way to check for runtime MTE support (as opposed to a HWCAP2-based >>>> check) in a traced process. >>>> >>>> Originally we were going to do it via empty-parameter ptrace calls, but you >>>> had mentioned something about a proc-based method, if I'm not mistaken. >>> >>> We could expose more information via proc_pid_arch_status() but that >>> would be the tagged address ABI and tag check fault mode and intended >>> for human consumption mostly. We don't have any ptrace interface that >>> exposes HWCAPs. Since the gdbserver runs on the same machine as the >>> debugged process, it can check the HWCAPs itself, they are the same for >>> all processes. >> >> Sorry, I think i haven't made it clear. I already have access to HWCAP2 both >> from GDB's and gdbserver's side. But HWCAP2 only indicates the availability >> of a particular feature in a CPU, it doesn't necessarily means the traced >> process is actively using MTE, right? > > Right, but "actively" is not well defined either. The only way to tell > whether a process is using MTE is to look for any PROT_MTE mappings. You > can access these via /proc//maps. In theory, one can use MTE > without enabling the tagged address ABI or even tag checking (i.e. no > prctl() call). > I see the problem. I was hoping for a more immediate form of runtime check. One debuggers would validate and enable all the tag checks and register access at process attach/startup. With that said, checking for PROT_MTE in /proc//maps may still be useful, but a process with no immediate PROT_MTE maps doesn't mean such process won't attempt to use PROT_MTE later on. I'll have to factor that in, but I think it'll work. I guess HWCAP2_MTE will be useful after all. We can just assume that whenever we have HWCAP2_MTE, we can fetch MTE registers and check for PROT_MTE. >> So GDB/gdbserver would need runtime checks to be able to tell if a process >> is using MTE, in which case the tools will pay attention to tags and >> additional MTE-related registers (sctlr and gcr) we plan to make available >> to userspace. > > I'm happy to expose GCR_EL1.Excl and the SCTLR_EL1.TCF0 bits via ptrace > as a thread state. The tags, however, are a property of the memory range > rather than a per-thread state. That's what makes it different from > other register-based features like SVE. That's my understanding as well. I'm assuming, based on our previous discussion, that we'll have those couple registers under a regset (maybe NT_ARM_MTE). > >> The original proposal was to have GDB send PTRACE_PEEKMTETAGS with a NULL >> address and check the result. Then GDB would be able to decide if the >> process is using MTE or not. > > We don't store this information in the kernel as a bool and I don't > think it would be useful either. I think gdb, when displaying memory, > should attempt to show tags as well if the corresponding range was > mapped with PROT_MTE. Just probing whether a thread ever used MTE > doesn't help since you need to be more precise on which address supports > tags. Thanks for making this clear. Checking with ptrace won't work then. It seems like /proc//maps is the way to go. > >>> BTW, in my pre-v4 patches (hopefully I'll post v4 this week), I changed >>> the ptrace tag access slightly to return an error (and no tags copied) >>> if the page has not been mapped with PROT_MTE. The other option would >>> have been read-as-zero/write-ignored as per the hardware behaviour. >>> Either option is fine by me but I thought the write-ignored part would >>> be more confusing for the debugger. If you have any preference here, >>> please let me know. >> >> I think erroring out is a better alternative, as long as the debugger can >> tell what the error means, like, for example, "this particular address >> doesn't make use of tags". > > And you could use this for probing whether the range has tags or not. > With my current patches it returns -EFAULT but happy to change this to > -EOPNOTSUPP or -EINVAL. Note that it only returns an error if no tags > copied. If gdb asks for a range of two pages and only the first one has > PROT_MTE, it will return 0 and set the number of tags copied equivalent > to the first page. A subsequent call would return an error. > > In my discussion with Dave on the documentation patch, I thought retries > wouldn't be needed but in the above case it may be useful to get an > error code. That's unless we change the interface to return an error and > also update the user iovec structure. > Let me think about this for a bit. I'm trying to factor in the /proc//maps contents. If debuggers know which pages have PROT_MTE set, then we can teach the tools not to PEEK/POKE tags from/to those memory ranges, which simplifies the error handling a bit.