Re: [PATCH v3] ARM: breakpoint: CFI breakpoints only on demand

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

On Wed, Jul 01, 2026 at 12:42:09PM +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, we fall through to return 1 from
> hw_breakpoint_pending() "unhandled fault" so userspace can make use
> of this breakpoint.
> 
> This of course does not work if userspace want to use CFI and custom
> breakpoints at the same time, and CONFIG_CFI does exist as something
> users might want to select for their kernel. If this is not good
> acceptable we need to think about other ways for CFI to interfer, such
> as not using BKPT at all (rather something like BUG()) and back out
> the offending patch until the compiler behaviour has changed.
> 
> Fixes: c3f89986fde7 ("ARM: 9391/2: hw_breakpoint: Handle CFI breakpoints")
> Reported-by: slipher <slipher@protonmail.com>
> Closes: https://lore.kernel.org/lkml/kJqktbpLphg_Pk5I5SPptgTLjl3E3eq5mN5UzCslyFj7Q1Irp-wDid4mj5eQVd2iZtRGXgeZd8goq195EkXdjyt864YMc8mVb2B9NGH91NQ=@protonmail.com/
> Signed-off-by: Linus Walleij <linusw@kernel.org>
> ---
> Trying to solve the CFI bug. Let's see of this first
> approach is acceptable for the reporter.

Looks fine to me, but will depend whether the reporter has CONFIG_CFI
enabled in their kernel build.

Have the LLVM compiler people responded to this bug yet? What is their
plan with the silly choice of BKPT usage for CFI failure?

-- 
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