[ltt-dev] [UST] Text relocations in 0.12

[ltt-dev] [UST] Text relocations in 0.12

[Jon Bernard]

As I prepared the 0.12 release for Debian, I noticed that libust.so was
not PIC:

    $ readelf -d libust/.libs/libust.so.0.0.0 | grep TEXTREL
    0x0000000000000016 (TEXTREL)            0x0

Since all of the objects in libust are built with -fPIC, I thought
perhaps there was some assembly added between 0.11 and 0.12 that
contained text relocations. I bisected the two tags down to the
offending commit:

    eb5d20c68aaf73661ffc02ba8fea3683c0358702

Within that commit, it seems to be a problem in include/ust/marker.h
with these lines:

@@ -129,7 +124,12 @@ struct marker {
[...]
+                       /*".section __markers_ptrs\n\t"*/ \
+                       ".section __markers_ptrs,\"a\", at progbits\n\t"                           \

If I make the __markers_ptrs section writable, with:

                        ".section __markers_ptrs,\"aw\"\n\t

TEXTREL goes away and everything seems okay, tests pass. Is this
a correct solution? I don't understand why the section must be writable
to avoid relocations, can anyone explain?

Cheers

Jon

[ltt-dev] [UST] Text relocations in 0.12

[Mathieu Desnoyers]

* Jon Bernard (jbernard at debian.org) wrote:
> As I prepared the 0.12 release for Debian, I noticed that libust.so was
> not PIC:
> 
>     $ readelf -d libust/.libs/libust.so.0.0.0 | grep TEXTREL
>     0x0000000000000016 (TEXTREL)            0x0
> 
> Since all of the objects in libust are built with -fPIC, I thought
> perhaps there was some assembly added between 0.11 and 0.12 that
> contained text relocations. I bisected the two tags down to the
> offending commit:
> 
>     eb5d20c68aaf73661ffc02ba8fea3683c0358702
> 
> Within that commit, it seems to be a problem in include/ust/marker.h
> with these lines:
> 
> @@ -129,7 +124,12 @@ struct marker {
> [...]
> +                       /*".section __markers_ptrs\n\t"*/ \
> +                       ".section __markers_ptrs,\"a\", at progbits\n\t"                           \
> 
> If I make the __markers_ptrs section writable, with:
> 
>                         ".section __markers_ptrs,\"aw\"\n\t
> 
> TEXTREL goes away and everything seems okay, tests pass. Is this
> a correct solution? I don't understand why the section must be writable
> to avoid relocations, can anyone explain?

Oh.. I think I see. __markers_ptrs contains pointers to another section
that must be populated by the dynamic linker (thus at runtime). If the
section is read-only, the linker cannot update them at load time, so a
relocation table is probably needed.

I'll fix this in the git tree. It will be there in the next release.

Thanks!

Mathieu

> 
> Cheers
> 
> Jon
> 
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> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com