[PATCH v7 8/9] ARM: vdso initialization, mapping, and synchronization

On 07/02/2014 10:54 AM, Andy Lutomirski wrote:

Caveat 2: (major) I'm kind of surprised that this, or the current
code, works reliably.  You're doing something that I tried briefly for
x86_64:

        _end = .;
        PROVIDE(end = .);

        . = ALIGN(PAGE_SIZE);
        PROVIDE(_vdso_data = .);

This sounds great, except that you're assuming that vdso_end -
vdso_start == ALIGN(_end, PAGE_SIZE) - (vdso base address).

If you *fully* strip the vdso (eu-strip --strip-sections), then this
is true: eu-strip --strip-sections outputs just the PT_LOAD piece of
the vdso.  But any binutils-generated incompletely stripped ELF image
contains a section table and possible non-allocatable sections at the
end.  If these exceed the amount of unused space in the last PT_LOAD
page, then they'll spill into the next page, and _vdso_data in the
vdso will no longer match the address at which vdso.c loads it.  Boom!

I bet you're getting away with this because the whole arm64 vdso seems
to be written in assembly, so it seems extremely unlikely to exceed
one page minus a few hundred bytes.  But if you start adding
complexity, you might get unlucky.

This is why I switched (in v5) the proposed 32-bit ARM VDSO to place the
data page before the code -- adding -frecord-gcc-switches to the
compiler flags was enough to break it.

I meant to call Will's attention to it at the time for arm64's sake, but
I guess it slipped my mind... sorry.