Messages in this thread | | | Date | Wed, 20 May 2020 18:41:50 +0100 | From | Will Deacon <> | Subject | Re: arm64: Register modification during syscall entry/exit stop |
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Hi Keno,
On Tue, May 19, 2020 at 04:37:34AM -0400, Keno Fischer wrote: > > Yes, we inherited this from ARM and I think strace relies on it. In > > hindsight, it is a little odd, although x7 is a parameter register in the > > PCS and so it won't be live on entry to a system call. > > I'm not familiar with the PCS acronym, but I assume you mean the > calling convention? You have more faith in userspace than I do ;). For > example, cursory googling brought up this arm64 syscall definition in musl: > > https://github.com/bminor/musl/blob/593caa456309714402ca4cb77c3770f4c24da9da/arch/aarch64/syscall_arch.h
Hmm, does that actually result in the SVC instruction getting inlined? I think that's quite dangerous, since we document that we can trash the SVE register state on a system call, for example. I'm also surprised that the register variables are honoured by compilers if that inlining can occur.
> The constraints on those asm blocks allow the compiler to assume that > x7 is preserved across the syscall. If a ptracer accidentally modified it > (which is easy to do in the situations that I mentioned), it could > absolutely cause incorrect execution of the userspace program. > > > Although the examples you've > > listed above are interesting, I don't see why x7 is important in any of > > them (and we only support up to 6 system call arguments). > > It's not so much that x7 is important, it's that lying to the ptracer is > problematic, because it might remember that lie and act on it later. > I did run into exactly this problem, where my ptracer accidentally > changed the value of x7 and caused incorrect execution in the tracee > (now that incorrect execution happened to be an assertion, because > my application is paranoid about these kinds of issues, but it was > incorrect nevertheless) > > If it would be helpful, I can code up the syscall entry -> signal trap example > ptracer to have a concrete example.
I guess I'm more interested in situations where the compiler thinks x7 is live, yet we clobber it.
Will
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