[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