From bac15a72fcf0ed143dd959604d9317c4bb94160a Mon Sep 17 00:00:00 2001 From: "Maciej W. Rozycki" Date: Fri, 29 Jun 2018 17:10:43 +0100 Subject: elf: Accept absolute (SHN_ABS) symbols whose value is zero [BZ #23307] We have this condition in `check_match' (in elf/dl-lookup.c): if (__glibc_unlikely ((sym->st_value == 0 /* No value. */ && stt != STT_TLS) || ELF_MACHINE_SYM_NO_MATCH (sym) || (type_class & (sym->st_shndx == SHN_UNDEF)))) return NULL; which causes all !STT_TLS symbols whose value is zero to be silently ignored in lookup. This may make sense for regular symbols, however not for absolute (SHN_ABS) ones, where zero is like any value, there's no special meaning attached to it. Consequently legitimate programs fail, for example taking the `elf/tst-absolute-sym' test case, substituting 0 for 0x55aa in `elf/tst-absolute-sym-lib.lds' and then trying to run the resulting program we get this: $ .../elf/tst-absolute-sym .../elf/tst-absolute-sym: symbol lookup error: .../elf/tst-absolute-sym-lib.so: undefined symbol: absolute $ even though the symbol clearly is there: $ readelf --dyn-syms .../elf/tst-absolute-sym-lib.so | grep '\babsolute\b' 7: 00000000 0 NOTYPE GLOBAL DEFAULT ABS absolute $ The check for the zero value has been there since forever or commit d66e34cd4234/08162fa88891 ("Implemented runtime dynamic linker to support ELF shared libraries.") dating back to May 2nd 1995, and the problem triggers regardless of commit e7feec374c63 ("elf: Correct absolute (SHN_ABS) symbol run-time calculation [BZ #19818]") being present or not. Fix the issue then, by permitting `sym->st_value' to be 0 for SHN_ABS symbols in lookup. [BZ #23307] * elf/dl-lookup.c (check_match): Do not reject a symbol whose `st_value' is 0 if `st_shndx' is SHN_ABS. * elf/tst-absolute-zero.c: New file. * elf/tst-absolute-zero-lib.c: New file. * elf/tst-absolute-zero-lib.lds: New file. * elf/Makefile (tests): Add `tst-absolute-zero'. (modules-names): Add `tst-absolute-zero-lib'. (LDLIBS-tst-absolute-zero-lib.so): New variable. ($(objpfx)tst-absolute-zero-lib.so): New dependency. ($(objpfx)tst-absolute-zero: New dependency. --- elf/tst-absolute-zero-lib.lds | 1 + 1 file changed, 1 insertion(+) create mode 100644 elf/tst-absolute-zero-lib.lds (limited to 'elf/tst-absolute-zero-lib.lds') diff --git a/elf/tst-absolute-zero-lib.lds b/elf/tst-absolute-zero-lib.lds new file mode 100644 index 0000000000..2fa65dc3f9 --- /dev/null +++ b/elf/tst-absolute-zero-lib.lds @@ -0,0 +1 @@ +"absolute" = 0; -- cgit v1.2.3