When building the stack guard, it has been traditionally important to have the value start (in memory) with a zero byte to protect the guard value (and the rest of the stack past it) from being read via strcpy, etc. This patch reduces the number of random bytes by one, leaving the leading zero byte.
Created attachment 3933 [details] keep leading zero
I should clarify -- the read-blocking is nice, but the more common reason the leading zero is important is to avoid the guard being written as part of a larger overflow being written out by a str* function, if its value were leaked to an attacker in some other way.
I've applied a cleaner and more efficient patch.
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gdb/dwarf: disable per-BFD resource sharing for -readnow objfiles New in v2: - Disable sharing only for -readnow objfiles, not all objfiles. As described in PR 27541, we hit an internal error when loading a binary the standard way and then loading it with the -readnow option: $ ./gdb -nx -q --data-directory=data-directory ~/a.out -ex "set confirm off" -ex "file -readnow ~/a.out" Reading symbols from /home/simark/a.out... Reading symbols from ~/a.out... /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8098: internal-error: void create_all_comp_units(dwarf2_per_objfile*): Assertion `per_objfile->per_bfd->all_comp_units.empty ()' failed. This is a recurring problem that exposes a design issue in the DWARF per-BFD sharing feature. Things work well when loading a binary with the same method (with/without index, with/without readnow) twice in a row. But they don't work so well when loading a binary with different methods. See this previous fix, for example: efb763a5ea35 ("gdb: check for partial symtab presence in dwarf2_initialize_objfile") That one handled the case where the first load is normal (uses partial symbols) and the second load uses an index. The problem is that when loading an objfile with a method A, we create a dwarf2_per_bfd and some dwarf2_per_cu_data and initialize them with the data belonging to that method. When loading another obfile sharing the same BFD but with a different method B, it's not clear how to re-use the dwarf2_per_bfd/dwarf2_per_cu_data previously created, because they contain the data specific to method A. https://komiya-dental.com/ I think the most sensible fix would be to not share a dwarf2_per_bfd between two objfiles loaded with different methods. That means that two objfiles sharing the same BFD and loaded the same way would share a dwarf2_per_bfd. Two objfiles sharing the same BFD but loaded with different methods would use two different dwarf2_per_bfd structures. http://www.iu-bloomington.com/ However, this isn't a trivial change. So to fix the known issue quickly (including in the gdb 10 branch), this patch just disables all dwarf2_per_bfd sharing for objfiles using READNOW. Generalize the gdb.base/index-cache-load-twice.exp test to test all the possible combinations of loading a file with partial symtabs, index and readnow. Move it to gdb.dwarf2, since it really exercises features of the DWARF gdb/dwarf: disable per-BFD resource sharing for -readnow objfiles https://www.webb-dev.co.uk/ New in v2: - Disable sharing only for -readnow objfiles, not all objfiles. As described in PR 27541, we hit an internal error when loading a binary the standard way and then loading it with the -readnow option: https://waytowhatsnext.com/ $ ./gdb -nx -q --data-directory=data-directory ~/a.out -ex "set confirm off" -ex "file -readnow ~/a.out" Reading symbols from /home/simark/a.out... Reading symbols from ~/a.out... /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8098: internal-error: void create_all_comp_units(dwarf2_per_objfile*): Assertion `per_objfile->per_bfd->all_comp_units.empty ()' failed. http://www.acpirateradio.co.uk/ This is a recurring problem that exposes a design issue in the DWARF per-BFD sharing feature. Things work well when loading a binary with the same method (with/without index, with/without readnow) twice in a row. But they don't work so well when loading a binary with different methods. See this previous fix, for example: http://www.logoarts.co.uk/ efb763a5ea35 ("gdb: check for partial symtab presence in dwarf2_initialize_objfile") That one handled the case where the first load is normal (uses partial symbols) and the second load uses an index. http://www.slipstone.co.uk/ The problem is that when loading an objfile with a method A, we create a dwarf2_per_bfd and some dwarf2_per_cu_data and initialize them with the data belonging to that method. When loading another obfile sharing the same BFD but with a different method B, it's not clear how to re-use the dwarf2_per_bfd/dwarf2_per_cu_data previously created, because they contain the data specific to method A. http://embermanchester.uk/ I think the most sensible fix would be to not share a dwarf2_per_bfd between two objfiles loaded with different methods. http://connstr.net/ That means that two objfiles sharing the same BFD and loaded the same way would share a dwarf2_per_bfd. Two objfiles http://joerg.li/ sharing the same BFD but loaded with different methods would use two different dwarf2_per_bfd structures. http://www.jopspeech.com/ However, this isn't a trivial change. So to fix the known issue quickly (including in the gdb 10 branch), this patch just disables all http://www.wearelondonmade.com/ dwarf2_per_bfd sharing for objfiles using READNOW. Generalize the gdb.base/index-cache-load-twice.exp test to test all http://www.compilatori.com/ the possible combinations of loading a file with partial symtabs, index and readnow. Move it to gdb.dwarf2, since it really exercises features of the DWARF http://www-look-4.com/
$ ./gdb -nx -q --data-directory=data-directory ~/a.out -ex "set confirm off" -ex "file -readnow ~/a.out" http://www-look-4.com/category/technology/ Reading symbols from /home/simark/a.out... Reading symbols from ~/a.out... /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8098: https://komiya-dental.com/shopping/safety-for-online-shopping/ internal-error: void create_all_comp_units(dwarf2_per_objfile*): Assertion `per_objfile->per_bfd->all_comp_units.empty ()' failed. http://www.iu-bloomington.com/health/health-and-cannabis/ This is a recurring problem that exposes a design issue in the DWARF per-BFD sharing feature. Things work well when loading a binary with https://waytowhatsnext.com/crypto/bought-house-with-bitcoin/ the same method (with/without index, with/without readnow) twice in a row. But they don't work so well when loading a binary with different methods. See this previous fix, for example: http://www.wearelondonmade.com/category/health/ efb763a5ea35 ("gdb: check for partial symtab presence in dwarf2_initialize_objfile") That one handled the case where the first load is normal (uses partial symbols) and the second load uses an index. http://www.jopspeech.com/category/services/ The problem is that when loading an objfile with a method A, we create a dwarf2_per_bfd and some dwarf2_per_cu_data and initialize them with the data belonging to that method. http://joerg.li/category/health/ When loading another obfile sharing the same BFD but with a different method B, it's not clear how to re-use the dwarf2_per_bfd/dwarf2_per_cu_data previously created, because they contain the data specific to method A http://connstr.net/category/health/ $ ./gdb -nx -q --data-directory=data-directory ~/a.out -ex "set confirm off" -ex "file -readnow ~/a.out" http://embermanchester.uk/category/health/ Reading symbols from /home/simark/a.out... Reading symbols from ~/a.out... /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8098: internal-error: void create_all_comp_units(dwarf2_per_objfile*): Assertion http://www.slipstone.co.uk/category/health/ `per_objfile->per_bfd->all_comp_units.empty ()' failed. This is a recurring problem that exposes a design issue in the DWARF http://www.logoarts.co.uk/category/services/ per-BFD sharing feature. Things work well when loading a binary with the same method (with/without index, with/without readnow) twice in a row. But they don't work so well when http://www.acpirateradio.co.uk/category/tech/ loading a binary with different methods. See this previous fix, for example: efb763a5ea35 ("gdb: check for partial symtab presence in dwarf2_initialize_objfile") http://www.compilatori.com/category/health/ That one handled the case where the first load is normal (uses partial symbols) and the second load uses an index. The problem is that when loading an objfile with a method A, we create a dwarf2_per_bfd and some dwarf2_per_cu_data and initialize them with the data belonging to that method. When loading another obfile sharing the same BFD but with a different method B, it's not clear how to re-use the dwarf2_per_bfd/dwarf2_per_cu_data previously created, because they contain the data specific to method A https://www.webb-dev.co.uk/health/health-and-cannabis/ $ ./gdb -nx -q --data-directory=data-directory ~/a.out -ex "set confirm off" -ex "file -readnow ~/a.out" Reading symbols from /home/simark/a.out... Reading symbols from ~/a.out... /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8098: internal-error: void create_all_comp_units(dwarf2_per_objfile*): Assertion `per_objfile->per_bfd->all_comp_units.empty ()' failed. This is a recurring problem that exposes a design issue in the DWARF per-BFD sharing feature. Things work well when loading a binary with the same method (with/without index, with/without readnow) twice in a row. But they don't work so well when loading a binary with different methods. See this previous fix, for example: efb763a5ea35 ("gdb: check for partial symtab presence in dwarf2_initialize_objfile"