On Mon, Mar 15, 2021 at 12:10:10PM +0100, Peter Zijlstra wrote:
> 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 <t...@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.
Ooooh, modules don't have this. They still have regular
.static_call_sites sections, and *those* are unaligned.
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk
Inf Al
[16] .static_call_sites PROGBITS 0000000000000000 008aa1 0006f0 00 WA
0 0 1
And that goes *BOOM*.. Let me ses if I can figure out how to make
objtool align those sections.
