Re: [PATCH RFC 01/10] mm: add Kernel Electric-Fence infrastructure

From: Dmitry Vyukov
Date: Thu Sep 10 2020 - 16:52:52 EST

On Mon, Sep 7, 2020 at 3:41 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> + int "Number of guarded objects available"
> + default 255
> + range 1 65535
> + help
> + The number of guarded objects available. For each KFENCE object, 2
> + pages are required; with one containing the object and two adjacent
> + ones used as guard pages.

Hi Marco,

Wonder if you tested build/boot with KFENCE_NUM_OBJECTS=65535? Can a
compiler create such a large object?

> + int "Fault injection for stress testing"
> + default 0
> + depends on EXPERT
> + help
> + The inverse probability with which to randomly protect KFENCE object
> + pages, resulting in spurious use-after-frees. The main purpose of
> + this option is to stress-test KFENCE with concurrent error reports
> + and allocations/frees. A value of 0 disables fault injection.

I would name this differently. "FAULT_INJECTION" is already taken for
a different thing, so it's a bit confusing.
KFENCE_DEBUG_SOMETHING may be a better name.
It would also be good to make it very clear in the short description
that this is for testing of KFENCE itself. When I configure syzbot I
routinely can't figure out if various DEBUG configs detect user
errors, or enable additional unit tests, or something else.
Maybe it should depend on DEBUG_KERNEL as well?

> +/*
> + * Get the canary byte pattern for @addr. Use a pattern that varies based on the
> + * lower 3 bits of the address, to detect memory corruptions with higher
> + * probability, where similar constants are used.
> + */
> +#define KFENCE_CANARY_PATTERN(addr) ((u8)0xaa ^ (u8)((unsigned long)addr & 0x7))

(addr) in macro body

> + seq_con_printf(seq,
> + "kfence-#%zd [0x" PTR_FMT "-0x" PTR_FMT

PTR_FMT is only used in this file, should it be declared in report.c?

Please post example reports somewhere. It's hard to figure out all
details of the reporting/formatting.