Re: [PATCH v10 02/15] livepatch: avoid position-based search if `-z unique-symbol` is available

[Alexander Lobakin]

From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Date: Fri, 11 Feb 2022 09:41:30 -0800

> 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?

Yeah, sure.

> 
> > 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?

A few folks previously.

> 
> 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.

Agree, I just want this series to be as least invasive for
non-FG-KASLR builds as possible. And currently this flag make depmod
emit a bunch of harmless false-positive warnings, so I'd wait until
at least the series is accepted / I post a patch for depmod and it
gets accepted.

> 
> > +++ 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.

Right, but for both those chunks the main idea is to let the
compiler optimize-out the code non-actual for unique-symbol builds:

add/remove: 0/0 grow/shrink: 1/2 up/down: 3/-80 (-77)
Function                                     old     new   delta
klp_find_callback                            139     142      +3
klp_find_object_symbol.cold                   85      48     -37
klp_find_object_symbol                       168     125     -43

> 
> -- 
> Josh

Thanks,
Al