From: Alexander Lobakin <alexandr.lobakin@intel.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Alexander Lobakin <alexandr.lobakin@intel.com>,
linux-hardening@vger.kernel.org, x86@kernel.org,
Borislav Petkov <bp@alien8.de>,
Jesse Brandeburg <jesse.brandeburg@intel.com>,
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llvm@lists.linux.dev
Subject: Re: [PATCH v10 02/15] livepatch: avoid position-based search if `-z unique-symbol` is available
Date: Mon, 14 Feb 2022 13:14:47 +0100 [thread overview]
Message-ID: <20220214121447.288695-1-alexandr.lobakin@intel.com> (raw)
In-Reply-To: <20220211174130.xxgjoqr2vidotvyw@treble>
From: Josh Poimboeuf <jpoimboe@redhat.com>
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
next prev parent reply other threads:[~2022-02-14 12:15 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-09 18:57 [PATCH v10 00/15] Function Granular KASLR Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 01/15] modpost: fix removing numeric suffixes Alexander Lobakin
2022-05-03 0:57 ` Masahiro Yamada
2022-05-03 7:31 ` Petr Mladek
2022-05-23 18:04 ` Masahiro Yamada
2022-05-24 11:33 ` Alexander Lobakin
2022-05-24 13:40 ` Masahiro Yamada
2022-02-09 18:57 ` [PATCH v10 02/15] livepatch: avoid position-based search if `-z unique-symbol` is available Alexander Lobakin
2022-02-11 17:41 ` Josh Poimboeuf
2022-02-11 18:05 ` Fāng-ruì Sòng
2022-02-11 18:35 ` Josh Poimboeuf
2022-02-14 12:24 ` Alexander Lobakin
2022-02-14 18:10 ` Josh Poimboeuf
2022-02-16 20:32 ` Joe Lawrence
2022-02-16 22:13 ` Josh Poimboeuf
2022-02-16 15:15 ` Miroslav Benes
2022-02-16 20:01 ` Josh Poimboeuf
2022-02-18 16:31 ` Alexander Lobakin
2022-02-18 20:08 ` Josh Poimboeuf
2022-02-14 12:14 ` Alexander Lobakin [this message]
2022-02-14 18:57 ` Josh Poimboeuf
2022-02-16 15:06 ` Miroslav Benes
2022-02-16 19:57 ` Josh Poimboeuf
2022-02-17 7:45 ` Miroslav Benes
2022-02-09 18:57 ` [PATCH v10 03/15] kallsyms: randomize /proc/kallsyms output order Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 04/15] arch: introduce asm function sections Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 05/15] x86: support " Alexander Lobakin
2022-02-11 15:45 ` Peter Zijlstra
2022-02-14 11:49 ` Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 06/15] x86: decouple ORC table sorting into a separate file Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 07/15] Makefile: add config options and build scripts for FG-KASLR Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 08/15] x86/tools: Add relative relocs for randomized functions Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 09/15] x86: Add support for function granular KASLR Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 10/15] FG-KASLR: use a scripted approach to handle .text.* sections Alexander Lobakin
2022-02-11 15:37 ` Peter Zijlstra
2022-02-14 11:34 ` Alexander Lobakin
2022-02-14 11:59 ` Peter Zijlstra
2022-02-14 12:30 ` Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 11/15] x86/boot: allow FG-KASLR to be selected Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 12/15] module: add arch-indep FG-KASLR for randomizing function layout Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 13/15] module: use a scripted approach for FG-KASLR Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 14/15] Documentation: add documentation " Alexander Lobakin
2022-02-09 18:57 ` [PATCH v10 15/15] maintainers: add MAINTAINERS entry " Alexander Lobakin
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