On Wed, Nov 07, 2018 at 11:08:56AM -0600, Josh Poimboeuf wrote: > On Wed, Nov 07, 2018 at 03:05:59PM +0100, Artem Savkov wrote: > > The way it is currently done it is possible for read_symbols() to find the > > same symbol as parent for ".cold" functions. > > I seem to remember having this discussion for kpatch-build, but I forget > the details. Can you explain how this can happen (and also add that > detail to the commit message)?
find_symbol_by_name() traverses the same lists as read_symbols and when we change sym->name in place without copying it it changes in the list as well. So if child function is before parent in sec->symbol_list the same function will be returned as "parent". It is hard for me to put it into words worthy to be included into commit message. > I haven't seen any bug reports, is it purely theoretical? No, 4.20-rc1 (actually anything after 4a60aa05a063 "objtool: Support per-function rodata sections", before that add_switch_table() doesn't seem to be called for those .cold. funcs) fails to build for mewith KCFLAGS="-ffunction-sections -fdata-sections" because objtool is segfaulting. > > This leads to a bunch of > > complications such as func length being set to 0 and a segfault in > > add_switch_table(). Fix by copying the search string instead of modifying > > it in place. > > > > Signed-off-by: Artem Savkov <[email protected]> > > --- > > tools/objtool/elf.c | 7 ++++--- > > 1 file changed, 4 insertions(+), 3 deletions(-) > > > > diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c > > index 6dbb9fae0f9d..781c8afb29b9 100644 > > --- a/tools/objtool/elf.c > > +++ b/tools/objtool/elf.c > > @@ -298,6 +298,7 @@ static int read_symbols(struct elf *elf) > > /* Create parent/child links for any cold subfunctions */ > > list_for_each_entry(sec, &elf->sections, list) { > > list_for_each_entry(sym, &sec->symbol_list, list) { > > + char *pname; > > if (sym->type != STT_FUNC) > > continue; > > sym->pfunc = sym->cfunc = sym; > > @@ -305,9 +306,9 @@ static int read_symbols(struct elf *elf) > > if (!coldstr) > > continue; > > > > - coldstr[0] = '\0'; > > - pfunc = find_symbol_by_name(elf, sym->name); > > - coldstr[0] = '.'; > > + pname = strndup(sym->name, coldstr - sym->name); > > + pfunc = find_symbol_by_name(elf, pname); > > + free(pname); > > > > if (!pfunc) { > > WARN("%s(): can't find parent function", > > -- > > 2.17.2 > > > > -- > Josh -- Regards, Artem
