Re: vdso.so mislinked by buggy linker was Re: Linus 2.6.23-rc1

Jakub Jelinek
2007-07-23T12:45:37Z

On Mon, Jul 23, 2007 at 01:56:20AM +0200, Andi Kleen wrote:
> On Monday 23 July 2007 01:38:40 Andre Noll wrote:
> [readded linux-kernel, Linus]
>
> > [Nr] Name Type Address Offset
> > Size EntSize Flags Link Info Align
> > [ 0] NULL 0000000000000000 00000000
> > 0000000000000000 0000000000000000 0 0 0
> > [ 1] .hash HASH ffffffffff700120 00000120
> > 00000000000000b4 0000000000000004 A 2 0 8
> > [ 2] .dynsym DYNSYM ffffffffff7001d8 000001d8
> > 0000000000000270 0000000000000018 A 3 12 8
> > [ 3] .dynstr STRTAB ffffffffff700448 00000448
> > 0000000000000059 0000000000000000 A 0 0 1
> > [ 4] .gnu.version VERSYM ffffffffff7004a2 000004a2
> > 0000000000000034 0000000000000002 A 2 0 2
> > [ 5] .gnu.version_d VERDEF ffffffffff7004d8 000004d8
> > 0000000000000038 0000000000000000 A 3 2 8
> > [ 6] .text PROGBITS ffffffffff700c00 00100bab
> ^^^^^^^^
> > 00000000000002e4 0000000000000000 AX 0 0 64
>
> It puts .text at 1MB. Your vdso file must be huge?
>
> It looks like it ignores the
> -Wl,-z,max-page-size=4096 -Wl,-z,common-page-size=4096
> options passed to it. The AMD64 ABI has a 1MB minimum page size, but
> these options are supposed to disable it.
>
> Not sure how to work around this, but having an 1+MB vdso would be incredibly
> wasteful. What version is it? Perhaps we just drop support for this. I can't
> think of a workaround currently.

Looking at vdso.lds.S, if you change just VDSO_TEXT_OFFSET to 0xc00 and
don't tweak the linker script, then you jump backwards with the dot, you
should even get a linker warning about it:

. = VDSO_PRELINK + VDSO_TEXT_OFFSET;

.text : { *(.text) } :text
.text.ptr : { *(.text.ptr) } :text
. = VDSO_PRELINK + 0x900;

Guess that 0x900 should have been VDSO_TEXT_OFFSET + 0x400 or something
similar. Also note that it is highly desirable to fit the whole vdso into
one page, so increasing VDSO_TEXT_OFFSET etc. offsets too much is just
wasting memory. From the above dump, VDSO_TEXT_OFFSET 0x500 is too low,
but 0x600 should work, assuming .data section is moved 0x100 higher as well.

Jakub