[PATCH v7 8/9] ARM: vdso initialization, mapping, and synchronization
Will Deacon
will.deacon at arm.com
Wed Jul 2 10:24:43 PDT 2014
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:
> >> > 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.
> >
> > 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:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: DYN (Shared object file)
Machine: AArch64
Version: 0x1
Entry point address: 0x2d0
Start of program headers: 64 (bytes into file)
Start of section headers: 1888 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 56 (bytes)
Number of program headers: 4
Size of section headers: 64 (bytes)
Number of section headers: 14
Section header string table index: 13
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .hash HASH 0000000000000120 00000120
0000000000000030 0000000000000004 A 2 0 8
[ 2] .dynsym DYNSYM 0000000000000150 00000150
00000000000000a8 0000000000000018 A 3 2 8
[ 3] .dynstr STRTAB 00000000000001f8 000001f8
0000000000000077 0000000000000000 A 0 0 1
[ 4] .gnu.version VERSYM 0000000000000270 00000270
000000000000000e 0000000000000002 A 2 0 2
[ 5] .gnu.version_d VERDEF 0000000000000280 00000280
0000000000000038 0000000000000000 A 3 2 8
[ 6] .note NOTE 00000000000002b8 000002b8
0000000000000018 0000000000000000 A 0 0 4
[ 7] .text PROGBITS 00000000000002d0 000002d0
0000000000000220 0000000000000000 AX 0 0 16
[ 8] .eh_frame_hdr PROGBITS 00000000000004f0 000004f0
0000000000000034 0000000000000000 A 0 0 4
[ 9] .eh_frame PROGBITS 0000000000000528 00000528
00000000000000b0 0000000000000000 A 0 0 8
[10] .dynamic DYNAMIC 00000000000005d8 000005d8
00000000000000f0 0000000000000010 WA 3 0 8
[11] .got PROGBITS 00000000000006c8 000006c8
0000000000000008 0000000000000008 WA 0 0 8
[12] .got.plt PROGBITS 00000000000006d0 000006d0
0000000000000018 0000000000000008 WA 0 0 8
[13] .shstrtab STRTAB 0000000000000000 000006e8
0000000000000078 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
LOAD 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x00000000000006e8 0x00000000000006e8 R E 10
DYNAMIC 0x00000000000005d8 0x00000000000005d8 0x00000000000005d8
0x00000000000000f0 0x00000000000000f0 R 8
NOTE 0x00000000000002b8 0x00000000000002b8 0x00000000000002b8
0x0000000000000018 0x0000000000000018 R 4
GNU_EH_FRAME 0x00000000000004f0 0x00000000000004f0 0x00000000000004f0
0x0000000000000034 0x0000000000000034 R 4
Section to Segment mapping:
Segment Sections...
00 .hash .dynsym .dynstr .gnu.version .gnu.version_d .note .text .eh_frame_hdr .eh_frame .dynamic .got .got.plt
01 .dynamic
02 .note
03 .eh_frame_hdr
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