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If you load a core file into GDB with the --write option, or "set write on" (equivalent), and then poke memory expecting it to patch the core binary, you'll notice something odd -- the write seems to succeed, but in reality, it doesn't. The value you wrote doesn't persist. Like so: $ gdb -q --write -c testsuite/outputs/gdb.base/patch/gcore.test [New LWP 615986] Core was generated by `/home/pedro/gdb/build/gdb/testsuite/outputs/gdb.base/patch/patch'. Program terminated with signal SIGTRAP, Trace/breakpoint trap. #0 0x0000555555555131 in ?? () (gdb) p *(unsigned char *)0x0000555555555131 = 1 $1 = 1 '\001' (gdb) p *(unsigned char *)0x0000555555555131 $2 = 185 '\271' (gdb) Diffing hexdumps of before/after patching, reveals that a "0x1" was actually written somewhere in the file. The problem is that the "0x1" was written at the wrong offset in the file... That happens because _bfd_elf_set_section_contents does this to seek to the section's offset: pos = hdr->sh_offset + offset; if (bfd_seek (abfd, pos, SEEK_SET) != 0 || bfd_bwrite (location, count, abfd) != count) return false; ... and 'hdr->sh_offset' is zero, so we seek to just OFFSET, which is incorrect. The reason 'hdr->sh_offset' is zero is that kernel-generated core files normally don't even have a section header table (gdb-generated ones do, but that's more an accident than a feature), and indeed elf_core_file_p doesn't even try to read sections at all: /* Core files are simply standard ELF formatted files that partition the file using the execution view of the file (program header table) rather than the linking view. In fact, there is no section header table in a core file. The process status information (including the contents of the general register set) and the floating point register set are stored in a segment of type PT_NOTE. We handcraft a couple of extra bfd sections that allow standard bfd access to the general registers (.reg) and the floating point registers (.reg2). */ bfd_cleanup elf_core_file_p (bfd *abfd) Changing _bfd_elf_set_section_contents from: pos = hdr->sh_offset + offset; to: pos = section->filepos + offset; fixes it. If we do that however, the tail end of _bfd_elf_set_section_contents ends up as a copy of _bfd_generic_set_section_contents, so just call the latter, thus eliminating some duplicate code. New GDB testcase included, which exercises both patching an executable and patching a core file. Patching an executable already works without this fix, because in that case BFD reads in the sections table. Still, we had no testcase for that yet. In fact, we have no "set write on" testcases at all, this is the first one. Tested on x86-64 GNU/Linux, gdb, ld, binutils, and gas. Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=18227 Change-Id: I0f49f58b48aabab2e269f2959b8fd8a7fe36fdce
README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.