Messages in this thread | | | Date | Sat, 27 Mar 2021 13:01:01 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH 1/4] arm64: expose orig_x0 in the user_pt_regs structure |
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On Fri, Mar 26, 2021 at 05:35:19PM -0700, Andrei Vagin wrote: > On Fri, Mar 26, 2021 at 11:28 AM Catalin Marinas > <catalin.marinas@arm.com> wrote: > > > > On Mon, Mar 22, 2021 at 03:50:50PM -0700, Andrei Vagin wrote: > > > diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h > > > index 758ae984ff97..3c118c5b0893 100644 > > > --- a/arch/arm64/include/uapi/asm/ptrace.h > > > +++ b/arch/arm64/include/uapi/asm/ptrace.h > > > @@ -90,6 +90,7 @@ struct user_pt_regs { > > > __u64 sp; > > > __u64 pc; > > > __u64 pstate; > > > + __u64 orig_x0; > > > }; > > > > That's a UAPI change, likely to go wrong. For example, a > > ptrace(PTRACE_GETREGSET, pid, REGSET_GPR, data) would write past the end > > of an old struct user_pt_regs in the debugger. > > ptrace(PTRACE_GETREGSET, ...) receives iovec: > ptrace(PTRACE_GETREGSET, pid, NT_PRSTATUS, &iov) > > iov contains a pointer to a buffer and its size and the kernel fills > only the part that fits the buffer. > I think this interface was invented to allow extending structures > without breaking backward compatibility.
You are right here, it doesn't write past the end of the iov buffer. However, it's still an ABI change. An unaware program using a newer user_pt_regs but running on an older kernel may be surprised that the updated iov.len is smaller than sizeof (struct user_pt_regs).
Changing this structure also changes the core dump format, see ELF_NGREG and ELF_CORE_COPY_REGS. Maybe this doesn't matter much either since the ELF note would have size information but I'd prefer if we didn't modify this structure.
-- Catalin
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