Re: [REGRESSION] 32-bit ARM's BKPT instruction no longer works

[Russell King <linux@armlinux.org.uk>]

On Sun, Jun 21, 2026 at 11:41:03PM +0100, Russell King (Oracle) wrote:
> On Sun, Jun 21, 2026 at 09:53:17PM +0000, slipher wrote:
> > 
> > On Sunday, June 21st, 2026 at 3:19 PM, Russell King (Oracle) <linux@armlinux.org.uk> wrote:
> > 
> > > On Sun, Jun 21, 2026 at 07:15:27PM +0000, slipher wrote:
> > > > Consider the C program for 32-bit ARM architectures:
> > > >
> > > > int main() {
> > > > 	__asm__ __volatile__ ("BKPT");
> > > > 	return 0;
> > > > }
> > > >
> > > >
> > > > Expected behavior is that this raises SIGTRAP. Since Linux 6.10 this no
> > > > longer happens; instead execution perpetually resumes at the same
> > > > instruction, using 100% of CPU. It does not matter whether GDB is
> > > > attached. I have tested with an armv7l CPU, but I imagine any other
> > > > variants with the BKPT instruction would be equally affected.
> > > 
> > > Looking at the code, I doubt this has ever cleanly raised SIGTRAP (can
> > > you check whether it does in kernels without c3f89986fde please?)
> > > 
> > > What I suspect instead is you get an "Unhandled ... abort" instead
> > > and the program forcefully killed as hw_breakpoint_pending() would
> > > have ARM_DSCR_MOE(dscr) == 3, and the switch() would set ret = 1.
> > > That triggers the fault handlers in arch/arm/mm/fault.c to
> > > complain bitterly, and forced a SIGTRAP to the program to kill it
> > > off. No resumption from an unhandled trap is expected.
> > 
> > I have tested with a 6.6 kernel. All of that is correct, as detailed in
> > the aforementioned blog post, except the last sentence. The switch does
> > set ret = 1, thereby passing on the exception. The kernel complains,
> > with such lines in dmesg output:
> > 
> > [ 1547.164526] Unhandled prefetch abort: breakpoint debug exception (0x222) at 0x0001051c
> 
> This message is printed at Alert level. It's just not supposed to
> happen, and if anyone sees it, it means someone cocked up in the kernel
> and didn't provide the code to handle a fault that can be generated.
> 
> In these situations, the kernel's response is to try and keep the system
> running by delivering a signal that should result in the process being
> terminated. In this case, the hardware breakpoint code tells the
> generic code to deliver a SIGTRAP / TRAP_HWBKPT, and this will be
> delivered by force_sig_fault() after the noisy kernel message has been
> produced.
> 
> force_sig_fault() will unblock the signal and set the handler to
> default if it was blocked or ignored. The default action for SIGTRAP
> should be to generate a coredump and terminate the program.
> 
> > Indeed, it is not clean or efficient; the blog
> > (https://www.jwhitham.org/2015/04/the-mystery-of-fifteen-millisecond.html)
> > even has a proposed patch to improve the performance when raising
> > SIGTRAP. However, it is possible to catch the signal, and even resume
> > with something like this:
> > 
> > 
> > #include <ucontext.h>
> > #include <signal.h>
> > #include <stdio.h>
> > 
> > void handl(int a, siginfo_t *b, void *uc) {
> >         puts("caught SIGTRAP");
> >         ((ucontext_t*)uc)->uc_mcontext.arm_pc += 4;
> > }
> > 
> > int main() {
> >         struct sigaction s;
> >         s.sa_flags = SA_SIGINFO;
> >         s.sa_sigaction = handl;
> >         sigemptyset(&s.sa_mask);
> >         sigaction(SIGTRAP, &s, 0);
> >         puts("start");
> >         __asm__ __volatile__("BKPT");
> >         puts("resumed");
> >         return 0;
> > }
> > 
> > Re-testing, I realized there is a huge caveat: SIGTRAP is *not* raised
> > when running under a debugger! If GDB is attached, either of the C
> > programs above will repeatedly resume at the faulting instruction on
> > Linux 6.6, just as they will with the latest kernels. So the regression
> > only affects the perhaps-obscure case of using BKPT without any
> > intention of attaching a debugger, unless that worked in even-earlier
> > versions of Linux.
> 
> ... and while it's repeatedly raising the same fault, it's flooding the
> kernel console with Alert level messages telling you the fault hasn't
> been handled even on older kernels... yet you seem to be under the
> impression that this is supposed to work.
> 
> You are testing something that has never been tested before, and are
> hitting behaviour that isn't _supposed_ to be clean.
> 
> That said, the change of behaviour is wrong. If
> hw_breakpoint_cfi_handler() doesn't understand the reason its been
> called, it should cause the old behaviour (where the alert message
> is printed) to be actioned.
> 
> The issue over whether BKPT should correctly raise a SIGTRAP that
> is appropriately handled is an entirely separate issue, which I
> would regard as a feature request rather than a regression.
> 
> Let me put it slightly differently. BKPT in userspace hasn't been
> supported by the kernel, and the behaviour you've seen from the
> kernel is incidental to the kernel's abort handling - it is not
> by design.
> 
> Architecturally, BKPT is used with JTAG debuggers, causing the
> processor to enter debug mode so a JTAG debugger can do its
> stuff. There was some discussion ten years ago whether LLVM
> should use BKPT for setting software breakpoints, and it seems
> they decided against it because of interfering with JTAG
> debuggers. See https://reviews.llvm.org/D16853?id=46899#347119
> 
> Also see the linked discussion from that post, where using BKPT
> was discussed with gdb. Basically, if a hardware JTAG debugger is
> connected, BKPT goes straight to the hardware debugger not the
> kernel. However, note that the sourceware discussion is talking
> about Thumb2 rather than ARM, but the same will apply there.
> 
> In essence, the decision was to stick with the UDF instructions
> for software breakpoints handled by the kernel, and leave BKPT
> for hardware JTAG debuggers. Consequently, explicitly executing
> BKPT without a hardware JTAG debugger is unexpected, the results
> of which are not guaranteed.
> 
> Indeed, under older architectures, you'll get an undefined
> instruction exception and the program killed by a SIGILL not a
> SIGTRAP, because BKPT isn't architecturally defined there.

For further clarification, see the ARM Architecture Reference Manual,
DDI0100E, which introduced BKPT, page 114, but specifically page 115
which states in the notes:

"Hardware override
"Debug hardware in an implementation is specifically permitted to
override the normal behavior of the BKPT instruction. Because of
this, software must not use this instruction for purposes other than
those documented by the debug system being used (if any). In
particular, software cannot rely on the Prefetch Abort exception
occurring, unless either there is guaranteed to be no debug hardware
in the system or the debug system specifies that it will occur.

"For more information, consult the documentation for the debug
system being used."

DDI0406C also mentions C2.2 states that if DBGEN is enabled, then
all debug events become halting and cause the CPU to enter debug
state (for a hardware debugger to respond to.) However, the above
statement is no longer present, but is covered via other means.
Indeed, a JTAG hardware debugger can still override BKPT to
put the CPU into debug mode and omit to generate the Prefetch
Abort exception.

Thus, BKPT isn't guaranteed to raise a prefetch abort depending
on whether there's a hardware debugger connected and how that
debugger has configured the interface.

-- 
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