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SubjectRe: [PATCH v2] kernel: add kcov code coverage
On Fri, Jan 15, 2016 at 03:07:59PM +0100, Dmitry Vyukov wrote:
> On Fri, Jan 15, 2016 at 2:05 PM, Andrey Ryabinin <ryabinin.a.a@gmail.com> wrote:
> > 2016-01-14 17:30 GMT+03:00 Dmitry Vyukov <dvyukov@google.com>:
> >> On Thu, Jan 14, 2016 at 11:50 AM, Andrey Ryabinin
> >> <ryabinin.a.a@gmail.com> wrote:
> >>> 2016-01-13 15:48 GMT+03:00 Dmitry Vyukov <dvyukov@google.com>:
> >>>
> >>>> + /* Read number of PCs collected. */
> >>>> + n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
> >>>> + /* PCs are shorten to uint32_t, so we need to restore the upper part. */
> >>>> + for (i = 0; i < n; i++)
> >>>> + printf("0xffffffff%0lx\n", (unsigned long)cover[i + 1]);
> >>
> >> Thanks for the review!
> >> Mailed v3 with fixes.
> >> Comments inline.
> >>
> >>> This works only for x86-64.
> >>> Probably there is no simple way to make this arch-independent with
> >>> 32-bit values.
> >>
> >> We probably could add an ioctl that returns base of the stripped PCs.
> >
> > You forgot about modules. With stripped PCs you'll start mixing
> > kernel's and module's PC (if distance between module and kernel > 4G).
>
> It's just that on x86 text and modules are within 4GB.
>
> I've checked that on arm64 it also seems to be the case:
>
> 48 * The module space lives between the addresses given by TASK_SIZE
> 49 * and PAGE_OFFSET - it must be within 128MB of the kernel text.
> 50 */
> 54 #define MODULES_END (PAGE_OFFSET)
> 55 #define MODULES_VADDR (MODULES_END - SZ_64M)

This won't necessarily remain true. With kASLR [1,2] the modules and
kernel might be located anywhere in the vmalloc area, independently.
Using PLTs removes the +/-128MB restriction, so they may be placed
anywhere in the vmalloc area.

On my defconfig kernel (4KiB, 39-bit VA) today, that area is ~246GiB wide:

[ 0.000000] Virtual kernel memory layout:
[ 0.000000] vmalloc : 0xffffff8000000000 - 0xffffffbdbfff0000 ( 246 GB)
[ 0.000000] vmemmap : 0xffffffbdc0000000 - 0xffffffbfc0000000 ( 8 GB maximum)
[ 0.000000] 0xffffffbdc2000000 - 0xffffffbde8000000 ( 608 MB actual)
[ 0.000000] fixed : 0xffffffbffa7fd000 - 0xffffffbffac00000 ( 4108 KB)
[ 0.000000] PCI I/O : 0xffffffbffae00000 - 0xffffffbffbe00000 ( 16 MB)
[ 0.000000] modules : 0xffffffbffc000000 - 0xffffffc000000000 ( 64 MB)
[ 0.000000] memory : 0xffffffc000000000 - 0xffffffc980000000 ( 38912 MB)
[ 0.000000] .init : 0xffffffc000a00000 - 0xffffffc000a9c000 ( 624 KB)
[ 0.000000] .text : 0xffffffc000080000 - 0xffffffc000a00000 ( 9728 KB)
[ 0.000000] .data : 0xffffffc000a9c000 - 0xffffffc000b17a00 ( 495 KB)

Kernels can be built with a 48-bit VA (and potentially larger in future
with ARMv8.2-A or later [3]). The vmalloc area (and hence the maximum
distances between modules and kernel) will increase grow with the VA
range.

Thanks,
Mark.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-January/398527.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-January/398534.html
[3] https://community.arm.com/groups/processors/blog/2016/01/05/armv8-a-architecture-evolution

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