Re: [PATCH] ARM: enable interrupts when arm_notify_die() is handling user mode errors

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

On Thu, Jun 25, 2026 at 11:30:08AM +0200, Sebastian Andrzej Siewior wrote:
> On 2026-06-25 10:05:52 [+0100], Russell King wrote:
> > > for this but actual breakpoint handling might be broken or is it just
> > > me? But then your stack trace looks like mine so :/
> > 
> > ARM Linux doesn't use BKPT. BKPT was an instruction introduced by Arm
> > Ltd in ARMv5TE. Prior to this, we use a UDF instruction instead (we
> > had to pick something!) and gdb and other tools use that as a
> > breapoint.
> > 
> > Moreover, BKPT isn't guaranteed to trap to the kernel, especially when
> > there is a hardware debugger connected. In that case, DDI0100E states
> > that use of BKPT must be according to the instructions provided with
> > the hardware debugger. This makes BKPT unsuitable for use.
> 
> So you are saying this:
> 
> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> index e62cc4be5adf6..11ac69113eca2 100644
> --- a/arch/arm/mm/fault.c
> +++ b/arch/arm/mm/fault.c
> @@ -595,6 +595,16 @@ do_bad(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
>  	return 1;
>  }
>  
> +static int do_debug_event(unsigned long addr, unsigned int fsr,
> +			  struct pt_regs *regs)
> +{
> +	if (!user_mode(regs))
> +		return 1;
> +	local_irq_enable();
> +	ptrace_break(regs);
> +	return 0;
> +}
> +
>  struct fsr_info {
>  	int	(*fn)(unsigned long addr, unsigned int fsr, struct pt_regs *regs);
>  	int	sig;
> diff --git a/arch/arm/mm/fsr-2level.c b/arch/arm/mm/fsr-2level.c
> index f2be95197265d..bfd718f64020c 100644
> --- a/arch/arm/mm/fsr-2level.c
> +++ b/arch/arm/mm/fsr-2level.c
> @@ -46,7 +46,7 @@ static struct fsr_info fsr_info[] = {
>  static struct fsr_info ifsr_info[] = {
>  	{ do_bad,		SIGBUS,  0,		"unknown 0"			   },
>  	{ do_bad,		SIGBUS,  0,		"unknown 1"			   },
> -	{ do_bad,		SIGBUS,  0,		"debug event"			   },
> +	{ do_debug_event,	SIGBUS,  0,		"debug event"			   },
>  	{ do_bad,		SIGSEGV, SEGV_ACCERR,	"section access flag fault"	   },
>  	{ do_bad,		SIGBUS,  0,		"unknown 4"			   },
>  	{ do_translation_fault,	SIGSEGV, SEGV_MAPERR,	"section translation fault"	   },
> diff --git a/arch/arm/mm/fsr-3level.c b/arch/arm/mm/fsr-3level.c
> index d0ae2963656a6..96c1d45d20d9e 100644
> --- a/arch/arm/mm/fsr-3level.c
> +++ b/arch/arm/mm/fsr-3level.c
> @@ -34,7 +34,7 @@ static struct fsr_info fsr_info[] = {
>  	{ do_bad,		SIGBUS,  0,		"synchronous parity error (translation table walk" },
>  	{ do_bad,		SIGBUS,  0,		"unknown 32"			},
>  	{ do_bad,		SIGBUS,  BUS_ADRALN,	"alignment fault"		},
> -	{ do_bad,		SIGBUS,  0,		"debug event"			},
> +	{ do_debug_event,	SIGBUS,  0,		"debug event"			},
>  	{ do_bad,		SIGBUS,  0,		"unknown 35"			},
>  	{ do_bad,		SIGBUS,  0,		"unknown 36"			},
>  	{ do_bad,		SIGBUS,  0,		"unknown 37"			},
> 
> is not worth doing it? With this I can my little testcase working.

No, it isn't, because if you enable PERF_EVENTS then BKPT breaks.
hw_breakpoint.c claims this vector.

Moreover, in older architectures, FSR=2 means "Terminal exception"
which is defined as "This indicates that an irrecoverable fault has
occurred. The circumstances under which this can happen (if at all)
are IMPLEMENTATION DEFINED." - from DDI0100E (which includes
ARMv5TE). In DDI0100F, this encoding was changed to "Debug exception".

Hence, the above can not be unconditional.

Then, we also have that FSR=2 is generated for a number of different
reasons (including hardware debug events) which may trigger.

Also a hardware debugger (e.g. connected via JTAG) could decide to
pass a BKPT exception on, and that could happen from the kernel. I
believe LLVM CFI uses BKPT (see LinusW's commit c3f89986fde7 ("ARM:
9391/2: hw_breakpoint: Handle CFI breakpoints")

BKPT is a total mess.

> That would be exc_int3() from arch/x86/kernel/traps.c.
> Besides doing "notify_die(DIE_INT3, "int3", regs, 0, X86_TRAP_BP, SIGTRAP);"
> 
> it does cond_local_irq_enable() which enables the interrupts if they
> were enabled by the "caller", sends the signal (SIGTRAP).

I'm happy with that approach as far as interrupts go, but we can't
change the behaviour for FSR=2 again, beyond fixing LinusW's
commit (which has recently been reported as a regression.)

Note that the change which makes this raise a SIGTRAP rather than
SIGBUS when PERF_EVENTS=y could _also_ be reported as a regression
that we would have to fix, and making FSR=2 raise a SIGTRAP now
could very well invite that regression to be reported.

Essentially, I don't think we can "fix" BKPT to always raise SIGTRAP.
The BKPT instruction is something the kernel has never _officially_
supported.

-- 
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