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

[Andy Lutomirski]

On Wed, Jul 2, 2014 at 10:24 AM, Will Deacon <will.deacon at arm.com> wrote:
> On Wed, Jul 02, 2014 at 05:47:08PM +0100, Andy Lutomirski wrote:
>> On Wed, Jul 2, 2014 at 9:27 AM, Will Deacon <will.deacon at arm.com> wrote:
>> > On Wed, Jul 02, 2014 at 05:18:59PM +0100, Nathan Lynch wrote:
>> >> On 07/02/2014 10:54 AM, Andy Lutomirski wrote:
>> >
>> > Hmm, so I could definitely look at doing the same thing, but I don't know if
>> > we actually need to for arm64. As Andy points out, we're written entirely in
>> > assembly and we objcopy -S to create the vdso.so. I've dumped the headers
>> > below and everything appears to be PT_LOAD.
>>
>> Your dump doesn't show the location of the section and section string
>> tables themselves.  Try:
>>
>> eu-readelf -l -h -S whatever.so
>
> Thanks. See below.
>
> Will
>
> --->8
>
> ELF Header:
>   Start of section headers:          1888 (bytes into file)
>   Size of section headers:           64 (bytes)
>   Number of section headers:         14

That's 896 bytes for the section table, starting at offset 1888.

>   Section header string table index: 13


>
> Section Headers:
>   [13] .shstrtab         STRTAB           0000000000000000  000006e8
>        0000000000000078  0000000000000000           0     0     1

120 bytes of section headers strings, starting at offset 1768 (and not
allocatable -- no 'A' here).

>
> Program Headers:
>   Type           Offset             VirtAddr           PhysAddr
>                  FileSiz            MemSiz              Flags  Align
>   LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
>                  0x00000000000006e8 0x00000000000006e8  R E    10

The loadable segment is 1768 bytes long, starting at the beginning of
the file (and therefore the beginning of the mapping).

So you have a total of 1016 bytes of non-allocatable stuff at the end
that I've accounted for.  I assume that the whole file is 2784 bytes.

If you added text or data to bring PT_LOAD to between 3081 and 4095
bytes, then the section headers and/or section string table would
cross a page boundary.

--Andy