Re: [GIT pull] locking/urgent for v5.12-rc3

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Mar 15, 2021 at 09:33:45AM +0100, Peter Zijlstra wrote:
> On Sun, Mar 14, 2021 at 01:15:25PM -0700, Linus Torvalds wrote:
> > On Sun, Mar 14, 2021 at 8:40 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> > >
> > >  - A fix for the static_call mechanism so it handles unaligned
> > >    addresses correctly.
> > 
> > I'm not disputing the fix in any way, but why weren't the relocation
> > info and function start addresses mutually aligned?
> > 
> > Are we perhaps missing some .align directive somewhere?
> > 
> > Or am I missing something?
> 
> So I considered looking into that, but since carrying the flag on the
> absolute address is always correct I figured it was the more robust fix.
> 
> I suppose I can try and figure out where alignment went wobbly.

include/asm-generic/vmlinux.lds.h:

#define STATIC_CALL_DATA						\
	. = ALIGN(8);							\
	__start_static_call_sites = .;					\
	KEEP(*(.static_call_sites))					\
	__stop_static_call_sites = .;					\
	__start_static_call_tramp_key = .;				\
	KEEP(*(.static_call_tramp_key))					\
	__stop_static_call_tramp_key = .;

#ifndef RO_AFTER_INIT_DATA
#define RO_AFTER_INIT_DATA						\
	. = ALIGN(8);							\
	__start_ro_after_init = .;					\
	*(.data..ro_after_init)						\
	JUMP_TABLE_DATA							\
	STATIC_CALL_DATA						\
	__end_ro_after_init = .;
#endif

	.rodata           : AT(ADDR(.rodata) - LOAD_OFFSET) {		\
		__start_rodata = .;					\
		*(.rodata) *(.rodata.*)					\
		SCHED_DATA						\
		RO_AFTER_INIT_DATA	/* Read only after init */	\
		. = ALIGN(8);						\
		__start___tracepoints_ptrs = .;				\
		KEEP(*(__tracepoints_ptrs)) /* Tracepoints: pointer array */ \
		__stop___tracepoints_ptrs = .;				\
		*(__tracepoints_strings)/* Tracepoints: strings */	\
	}								\


$ nm defconfig-build/vmlinux | grep static_call_sites
ffffffff82916dc0 D __start_static_call_sites
ffffffff8291aaf0 D __stop_static_call_sites


Which all reads to me like it *SHOULD* work. Howver when I was debugging
the crash as reported by Steve (using his .config), I definitely saw
non-aligned base offsets causing mayhem.