On Tue, Feb 23, 2021 at 3:22 PM Martin Liška <mli...@suse.cz> wrote:
>
> On 2/23/21 12:56 PM, Richard Biener wrote:
> > Can't we fix the asan runtime? Does the same issue happen when merging
> > two comdat with different alignment and LTO?
>
> All right, there's a detail explanation what happens.
> Let's consider the following example:
>
> struct my_struct
> {
> unsigned long volatile x;
> } __attribute__((aligned(128)));
>
> static int array[5][6] = {};
> static struct my_struct variable128 = {1UL};
> static struct my_struct variable32 __attribute__((aligned(64))) = {1UL};
>
> Here we have 2 variables (variable128 and variable32) that are merged. Later
> on,
> we decide not to protect the global variable variable128 due to:
> || DECL_ALIGN_UNIT (decl) > 2 * ASAN_RED_ZONE_SIZE
>
> Without ICF we end up with:
>
> .align 64
> .type variable32, @object
> .size variable32, 128
> variable32:
> .zero 128
> .zero 32
> .align 128
> .type variable128, @object
> .size variable128, 128
> variable128:
> .zero 128
>
> As seen, variable32 has .zero 128+32, where 32 is the red-zone (and alignment
> is increased to 64).
>
> With ICF we end up with:
>
> .align 128
> .type variable128, @object
> .size variable128, 128
> variable128:
> .zero 128
> .set variable32,variable128
>
> So variable32 points to variable128 (which has no prepared red zone +
> alignment is the same).
> $ nm -n a.out
> ...
> 0000000000400b80 r variable128
> 0000000000400b80 r variable32
> 0000000000400c00 r array
>
> 0000000000400c00 - 0000000000400b80 == sizeof(variable32).
>
> Then we tell libasan what is the variable size and size of the corresponding
> red zone:
> $ ASAN_OPTIONS=report_globals=3 ./a.out
> ...
> ==20602==Added Global[0x000000403080]: beg=0x000000400b80 size=128/160
> name=variable32 module=asan.c dyn_init=0 odr_indicator=0x000000000000
Ah, so the issue is that ASAN still sees both variables (and isn't
properly cgraph/varpool aware)? So instead of just
keying on different alignment you'd have to verify in ICF whether the
decls are "registered the same" by ASAN, no?
Or simply not perform any variable ICF when ASAN is enabled?
> And bad thinks happen. So I really think ICF should not merge the variables.
> Please provide a comdat test-case :)
>
> Thanks,
> Martin
>
>
