On Fri, Mar 26, 2021 at 05:35:19PM -0700, Andrei Vagin wrote:

On Fri, Mar 26, 2021 at 11:28 AM Catalin Marinas
[off-list ref] wrote:

quoted

On Mon, Mar 22, 2021 at 03:50:50PM -0700, Andrei Vagin wrote:

quoted

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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