Re: [PATCH 20/24] objtool: Another static block fail

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jan 29, 2018 at 04:52:53PM -0600, Josh Poimboeuf wrote:
> On Tue, Jan 23, 2018 at 04:25:59PM +0100, Peter Zijlstra wrote:
> > I've observed GCC generate:
> > 
> >   sym:
> >      NOP/JMP 1f	(static_branch)
> >      JMP 2f
> >   1: /* crud */
> >      JMP 3f
> >   2: /* other crud */
> > 
> >   3: RETQ
> > 
> > 
> > This means we need to follow unconditional jumps; be conservative and
> > only follow if its a unique jump.
> > 
> > (I've not yet figured out which CONFIG option is responsible for this,
> >  a normal defconfig build does not generate crap like this)
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> 
> Any chance we can just add a compiler barrier to the assertion macro and
> avoid all this grow_static_blocks() mess?  It seems a bit... fragile.

It is all rather unfortunate yes.. :/ I've tried to keep the grow stuff
as conservative as possible while still covering all the weirdness I
found. And while it was great fun, I do agree it would be much better to
not have to do this.


You're thinking of something like this?

 static __always_inline void arch_static_assert(void)
 {
        asm volatile ("1:\n\t"
                      ".pushsection .discard.jump_assert \n\t"
                      _ASM_ALIGN  "\n\t"
                      _ASM_PTR "1b \n\t"
-                     ".popsection \n\t");
+                     ".popsection \n\t" ::: "memory");
 }

That doesn't seem to matter much; see here:

static void
ttwu_stat(struct task_struct *p, int cpu, int wake_flags)
{
        struct rq *rq;

        if (!schedstat_enabled())
                return;

        rq = this_rq();


$ objdump -dr build/kernel/sched/core.o

0000000000001910 <ttwu_stat>:
    1910:       e8 00 00 00 00          callq  1915 <ttwu_stat+0x5>
                        1911: R_X86_64_PC32     __fentry__-0x4
    1915:       41 57                   push   %r15
    1917:       41 56                   push   %r14
    1919:       41 55                   push   %r13
    191b:       41 54                   push   %r12
    191d:       55                      push   %rbp
    191e:       53                      push   %rbx
    191f:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
    1924:       eb 25                   jmp    194b <ttwu_stat+0x3b>
    1926:       41 89 d5                mov    %edx,%r13d
    1929:       41 89 f4                mov    %esi,%r12d
    192c:       48 89 fb                mov    %rdi,%rbx
    192f:       49 c7 c6 00 00 00 00    mov    $0x0,%r14
                        1932: R_X86_64_32S      runqueues


$ objdump -j __jump_table -sr build/kernel/sched.o

0000000000000048 R_X86_64_64       .text+0x000000000000191f
0000000000000050 R_X86_64_64       .text+0x0000000000001926
0000000000000058 R_X86_64_64       sched_schedstats


$ objdump -j .discard.jump_assert -dr build/kernel/sched.o

0000000000000000 R_X86_64_64       .text+0x000000000000192f


It still lifts random crud over that first initial statement (the rq
load).