Re: [PATCH v4] ARM: breakpoint: CFI breakpoints only on demand

[Nathan Chancellor]

On Fri, Jul 03, 2026 at 02:25:27PM +0200, Linus Walleij wrote:
> This removes the stub hw_breakpoint_cfi_handler() from ARM, making
> it not steal breakpoint type 0x03 (ARM_ENTRY_CFI_BREAKPOINT) unless
> CFI is actively used in the kernel.
> 
> When not instrumenting with CFI, or when a breakpoint is issued in
> userspace, we fall through to return 1 from hw_breakpoint_pending()
> "unhandled fault" so userspace can make use of this breakpoint.
> 
> Tested with LKDTM and this command line:
> echo CFI_FORWARD_PROTO > /sys/kernel/debug/provoke-crash/DIRECT
> still works as expected.
> 
> Fixes: c3f89986fde7 ("ARM: 9391/2: hw_breakpoint: Handle CFI breakpoints")
> Reported-by: slipher <slipher@xxxxxxxxxxxxxx>
> Suggested-by: Mark Rutland <mark.rutland@xxxxxxx>
> Closes: https://lore.kernel.org/lkml/kJqktbpLphg_Pk5I5SPptgTLjl3E3eq5mN5UzCslyFj7Q1Irp-wDid4mj5eQVd2iZtRGXgeZd8goq195EkXdjyt864YMc8mVb2B9NGH91NQ=@protonmail.com/
> Signed-off-by: Linus Walleij <linusw@xxxxxxxxxx>

Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>

-- 
Cheers,
Nathan