Messages in this thread | | | Date | Mon, 24 Oct 2022 12:18:26 +0100 | From | Mark Rutland <> | Subject | Re: kCFI && patchable-function-entry=M,N |
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On Fri, Oct 21, 2022 at 09:14:41PM -0700, Fangrui Song wrote: > On Fri, Oct 21, 2022 at 10:39 AM Sami Tolvanen <samitolvanen@google.com> wrote: > > > > On Fri, Oct 21, 2022 at 8:56 AM Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > Hi, > > > > > > For arm64, I'd like to use -fatchable-function-entry=M,N (where N > 0), for our > > > ftrace implementation, which instruments *some* but not all functions. > > > Unfortuntately, this doesn't play nicely with -fsanitize=kcfi, as instrumented > > > and non-instrumented functions don't agree on where the type hash should live > > > relative to the function entry point, making them incompatible with one another. > > > > Yes, the current implementation assumes that if prefix nops are used, > > all functions have the same number of them. > > > > > Is there any mechanism today that we could use to solve this, or could we > > > extend clang to have some options to control this behaviour? > > > > I don't think there's a mechanism to work around the issue right now, > > but we could just change where the hash is emitted on arm64. > > > > > It would also be helpful to have a symbol before both the hash and pre-function > > > NOPs so that we can filter those out of probes patching (I see that x86 does > > > this with the __cfi_function symbol). > > > > Adding a symbol before the hash isn't a problem, but if we move the > > hash and want the symbol to be placed before the prefix nops as well, > > we might need a flag to control this. Fangrui, what do you think? > > > > Sami > > Since the kcfi code expects the hash to appear in a specific location > so that an instrumented indirect jump can reliably obtain the hash. > For a translation unit `-fpatchable-function-entry=N,M` may be > specified or not, and we want both to work. Therefore, I agree that a > consistent hash location will help. This argument favors placing M > nops before the hash. The downside is a restriction on how the M nops > can be used. Previously if M>0, the runtime code needs to check > whether a BTI exists to locate the N-M after-function-entry NOPs. If > the hash appears after the M nops, the runtime code needs to > additionally knows whether the hash exists. My question is how to > reliably detect this.
That's a fair point.
For detecting BTI we can scan the binary for BTI/NOP at M instructions into the patch-site, but a similar approach won't be reliable for the type hash since the type hash itself could have the same bit pattern as an instruction.
> If there is motivation using M>0, I'd like to know the concrete code > sequence for `-fpatchable-function-entry=N,M` and how the runtime code > detects the NOPs with optional hash and optional BTI.
For the BTI case alone, I have code at:
https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=arm64/ftrace/per-callsite-ops&id=272a580fd5b7acc31747505d71530cee7cc2837d
... the gist being that it checks the instruction M insns after the start of the patch site.
For the type hash, I think there are a few options, e.g.
* Restricting the type hash to a set of values that can be identified (e.g. encoding those as a permanently-undefined UDF with a 16-bit immediate).
* Adding options to record additional metadata along with the pointer to the patch-site in the __patchable_function_entries section.
* Adding an option to record patch-site variants to sub-sections of the __patchable_function_entries section, so that at link time these can be grouped separately, e.g.
* __patchable_function_entries.??? // no BTI, no type hash * __patchable_function_entries.bti // has BTI * __patchable_function_entries.bti_cfi // has BTI and type hash * __patchable_function_entries.cfi // has type hash
Do any of those approaches sound plausible to you?
Thanks, Mark.
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