On Fri, Feb 11, 2022 at 9:41 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > On Wed, Feb 09, 2022 at 07:57:39PM +0100, Alexander Lobakin wrote: > > Position-based search, which means that if there are several symbols > > with the same name, the user needs to additionally provide the > > "index" of a desired symbol, is fragile. For example, it breaks > > when two symbols with the same name are located in different > > sections. > > > > Since a while, LD has a flag `-z unique-symbol` which appends > > numeric suffixes to the functions with the same name (in symtab > > and strtab). It can be used to effectively prevent from having > > any ambiguity when referring to a symbol by its name. > > In the patch description can you also give the version of binutils (and > possibly other linkers) which have the flag? GNU ld>=2.36 supports -z unique-symbol. ld.lld doesn't support -z unique-symbol. I subscribe to llvm@xxxxxxxxxxxxxxx and happen to notice this message (can't keep up with the changes...) I am a bit concerned with this option and replied last time on https://lore.kernel.org/r/20220105032456.hs3od326sdl4zjv4@xxxxxxxxxx My full reasoning is on https://maskray.me/blog/2020-11-15-explain-gnu-linker-options#z-unique-symbol > > Check for its availability and always prefer when the livepatching > > is on. It can be used unconditionally later on after broader testing > > on a wide variety of machines, but for now let's stick to the actual > > CONFIG_LIVEPATCH=y case, which is true for most of distro configs > > anyways. > > Has anybody objected to just enabling it for *all* configs, not just for > livepatch? > > I'd much prefer that: the less "special" livepatch is (and the distros > which enable it), the better. And I think having unique symbols would > benefit some other components. > > > +++ b/kernel/livepatch/core.c > > @@ -143,11 +143,13 @@ static int klp_find_callback(void *data, const char *name, > > args->count++; > > > > /* > > - * Finish the search when the symbol is found for the desired position > > - * or the position is not defined for a non-unique symbol. > > + * Finish the search when unique symbol names are enabled > > + * or the symbol is found for the desired position or the > > + * position is not defined for a non-unique symbol. > > */ > > - if ((args->pos && (args->count == args->pos)) || > > - (!args->pos && (args->count > 1))) > > + if (IS_ENABLED(CONFIG_LD_HAS_Z_UNIQUE_SYMBOL) || > > + (args->pos && args->count == args->pos) || > > + (!args->pos && args->count > 1)) > > return 1; > > There's no real need to do this. The code already works as-is, even if > there are no unique symbols. > > Even if there are no duplicates, there's little harm in going through > all the symbols anyway, to check for errors just in case something > unexpected happened with the linking (unexpected duplicate) or the patch > creation (unexpected sympos). It's not a hot path, so performance isn't > really a concern. > > When the old linker versions eventually age out, we can then go strip > out all the sympos stuff. > > > @@ -169,6 +171,13 @@ static int klp_find_object_symbol(const char *objname, const char *name, > > else > > kallsyms_on_each_symbol(klp_find_callback, &args); > > > > + /* > > + * If the LD's `-z unique-symbol` flag is available and enabled, > > + * sympos checks are not relevant. > > + */ > > + if (IS_ENABLED(CONFIG_LD_HAS_Z_UNIQUE_SYMBOL)) > > + sympos = 0; > > + > > Similarly, I don't see a need for this. If the patch is legit then > sympos should already be zero. If not, an error gets reported and the > patch fails to load. > > -- > Josh > > -- 宋方睿
