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When a binary has a PT_GNU_RELRO segment, the elfhack injected code uses mprotect to add the writable flag to relocated pages before applying relocations, removing it afterwards. To do so, the elfhack program uses the location and size of the PT_GNU_RELRO segment, and adjusts it to be aligned according to the PT_LOAD alignment. The problem here is that the PT_LOAD alignment doesn't necessarily match the actual page alignment, and the resulting mprotect may end up not covering the full extent of what the dynamic linker has protected read-only according to the PT_GNU_RELRO segment. In turn, this can lead to a crash on startup when trying to apply relocations to the still read-only locations. Practically speaking, this doesn't end up being a problem on x86, where the PT_LOAD alignment is usually 4096, which happens to be the page size, but on Debian armhf, it is 64k, while the run time page size can be 4k. --HG-- extra : rebase_source : 5ac7356f685d87c1628727e6c84f7615409c57a5 |
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| .. | ||
| inject | ||
| dummy.c | ||
| elf.cpp | ||
| elfhack.cpp | ||
| elfxx.h | ||
| inject.c | ||
| Makefile.in | ||
| moz.build | ||
| README | ||
| test-array.c | ||
| test-ctors.c | ||
| test.c | ||
Elfhack is a program to optimize ELF binaries for size and cold startup speed. Presently, it is quite experimental, though it works well for the target it was created for: Firefox's libxul.so. Elfhack currently only does one thing: packing dynamic relocations ; which ends up being a quite complex task, that can be summarized this way: - Remove RELATIVE relocations from the .rel.dyn/.rela.dyn section. - Inject a small code able to apply relative relocations "by hand" after the .rel.dyn/.rela.dyn section. - Inject a section containing relocative relocations in a different and more packed format, after the small code. - Register the small code as DT_INIT function. Make the small code call what was initially the DT_INIT function, if there was one. - Remove the hole between the new section containing relative relocations and the following sections, adjusting offsets and base addresses accordingly. - Adjust PT_LOAD entries to fit new offsets, and add an additional PT_LOAD entry when that is necessary to handle the discrepancy between offsets and base addresses, meaning the section offsets may yet again need adjustments. - Adjust various DT_* dynamic tags to fit the new ELF layout. - Adjust section headers. - Adjust ELF headers. See http://glandium.org/blog/?p=1177#relocations for some figures.