Re: [PATCH v2 1/3] arm64: ptrace: fix hw_break_set() by setting addr and ctrl together

[Will Deacon <will@kernel.org>]

On Wed, Aug 27, 2025 at 09:41:11AM +0800, Bill Tsui wrote:
> PTRACE_SETREGSET fails when setting a hardware breakpoint on a
> non-4-byte aligned address with a valid length to a 32-bit tracee. The
> length should be valid as long as the range started from the address
> is within one aligned 4 bytes.
> 
> The cause is that hw_break_set() modifies a breakpoint's addr
> first and then ctrl. This calls modify_user_hw_breakpoint() twice,
> although once registering both suffices. The first modification causes
> errors because new addr and old ctrl can be an invalid combination at
> hw_breakpoint_arch_parse(). For example, when a user sets a hardware
> breakpoint with addr=0x2 and ctrl.len=1, hw_breakpoint_arch_parse()
> will first see addr=0x2 and ctrl.len=4 (the default) and return
> -EINVAL. On the other hand, if a user sets the same value to
> a breakpoint whose ctrl.len has previously been set to 1 or 2,
> it succeeds.
> 
> The fix is to set addr and ctrl in one modify_user_hw_breakpoint(),
> effectively eliminating the discrepancy in validation.
> 
> Signed-off-by: Bill Tsui <b10902118@ntu.edu.tw>
> ---
>  arch/arm64/kernel/ptrace.c | 34 +++++++++++++++++++++++++++++-----
>  1 file changed, 29 insertions(+), 5 deletions(-)

Given that:

  (a) This is a pretty niche interface (primarily/exclusively (?) used
      by GDB)
  (b) It's been like this for a long time
  (c) Userspace can work around the problem

I'm not sure I see the benefit of trying to handle this differently
in the kernel.

If somebody _does_ have the time and energy for significant surgery
on this code, then the best thing to do would be to remove the
indirection through 'perf_events' altogether. I did make a start on
that once but it's a thankless job and I got preempted by other stuff.

Will