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Date: Wed, 24 Apr 2019 19:47:27 +0200 (CEST)
From: Miroslav Benes <mbenes@...e.cz>
To: Joe Lawrence <joe.lawrence@...hat.com>
cc: linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
linux-kbuild@...r.kernel.org, Jessica Yu <jeyu@...nel.org>,
Jiri Kosina <jikos@...nel.org>,
Joao Moreira <jmoreira@...e.de>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
Masahiro Yamada <yamada.masahiro@...ionext.com>,
Michael Matz <matz@...e.de>, Nicolai Stange <nstange@...e.de>,
Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v3 3/9] livepatch: Add klp-convert tool
On Tue, 23 Apr 2019, Joe Lawrence wrote:
> On Wed, Apr 10, 2019 at 11:50:52AM -0400, Joe Lawrence wrote:
> >
> > [ ... snip ... ]
> >
> > +static bool convert_rela(struct section *oldsec, struct rela *r,
> > + struct sympos *sp, struct elf *klp_elf)
> > +{
> > + struct section *sec;
> > + struct rela *r1, *r2;
> > +
> > + sec = get_or_create_klp_rela_section(oldsec, sp, klp_elf);
> > + if (!sec) {
> > + WARN("Can't create or access klp.rela section (%s.%s)\n",
> > + sp->object_name, sp->symbol_name);
> > + return false;
> > + }
> > +
> > + if (!convert_klp_symbol(r->sym, sp)) {
> > + WARN("Unable to convert symbol name (%s.%s)\n", sec->name,
> > + r->sym->name);
> > + return false;
> > + }
> > +
> > + /* Move the converted rela to klp rela section */
> > + list_for_each_entry_safe(r1, r2, &oldsec->relas, list) {
> > + if (r1->sym->name == r->sym->name) {
> > + list_del(&r1->list);
> > + list_add(&r1->list, &sec->relas);
> > + }
> > + }
> > + return true;
> > +}
>
> This one took a while to find and debug, but I believe that
> convert_rela()'s list removal is not as safe as it thinks it is.
>
> Start with its calling context from main() below:
>
> > + list_for_each_entry_safe(sec, aux, &klp_elf->sections, list) {
> > + if (!is_rela_section(sec))
> > + continue;
> > +
> > + list_for_each_entry_safe(rela, tmprela, &sec->relas, list) {
> > + if (!must_convert(rela->sym))
> > + continue;
> > +
> > + if (!find_missing_position(rela->sym, &sp)) {
> > + WARN("Unable to find missing symbol: %s",
> > + rela->sym->name);
> > + return -1;
> > + }
> > + if (!convert_rela(sec, rela, &sp, klp_elf)) {
> > + WARN("Unable to convert relocation: %s",
> > + rela->sym->name);
> > + return -1;
> > + }
> > + }
> > + }
>
> AFAIK the *_safe list traversals, they cache the ->next value at the
> beginning of each iteration, so that one could blow the current element
> in position away. The cached ->next value is then assigned when moving
> to the next iteration.
>
> But notice how convert_rela() looks through the entire list of
> relocations, moving each rela with a matching symbol?
>
> Consider a slight tweak to samples/livepatch-annotated.c:
>
>
> static int livepatch_cmdline_proc_show(struct seq_file *m, void *v)
> {
> + if (saved_command_line)
> + saved_command_line[0] = '\0';
> +
> seq_printf(m, "%s livepatch=1\n", saved_command_line);
> return 0;
> }
>
>
> On my system, this generates relocations like this:
>
> % eu-readelf --relocs samples/livepatch/livepatch-annotated-sample.o
> Relocation section [ 2] '.rela.text' for section [ 1] '.text' at offset 0x98 contains 9 entries:
> Offset Type Value Addend Name
> 0x0000000000000001 X86_64_PC32 000000000000000000 -4 __fentry__
> 0x0000000000000008 X86_64_PC32 000000000000000000 -4 saved_command_line
> 0x0000000000000017 X86_64_PC32 000000000000000000 -4 saved_command_line
> 0x000000000000001e X86_64_32S 000000000000000000 +0 .rodata.str1.1
> 0x0000000000000023 X86_64_PC32 000000000000000000 -4 seq_printf
> 0x0000000000000031 X86_64_PC32 000000000000000000 -4 __fentry__
> 0x0000000000000038 X86_64_32S 000000000000000000 +0 .data
> 0x0000000000000051 X86_64_PC32 000000000000000000 -4 __fentry__
> 0x000000000000003d X86_64_PC32 000000000000000000 -4 klp_enable_patch
>
> We now have back-to-back rela's with sym->name = "saved_command_line".
> When the first is converted, convert_rela() will move both of them to
> the klp rela section. The linked list values may be consistent, but the
> cached ->next value will be bogus and the in-flight-traversal will run
> off the rails.
Yep, valid, if I'm reading the code correctly.
> I think we can work around it with a combination of 1) only moving a
> single rela symbol at a time in convert_rela and 2) processing the
> second (third, etc.) a little bit more so that they are moved
> individually.
>
> I hacked this together and it works against the livepatch-annotate.c
> test above so far...
>
> -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8--
>
> diff --git a/scripts/livepatch/klp-convert.c b/scripts/livepatch/klp-convert.c
> index 82c27d219372..126395f1c0cd 100644
> --- a/scripts/livepatch/klp-convert.c
> +++ b/scripts/livepatch/klp-convert.c
> @@ -517,6 +517,7 @@ static bool convert_rela(struct section *oldsec, struct rela *r,
> if (r1->sym->name == r->sym->name) {
> list_del(&r1->list);
> list_add(&r1->list, &sec->relas);
> + break;
> }
> }
> return true;
Couldn't we remove the loop all together instead of breaking it?
list_del(&r->list);
list_add(&r->list, &sec->relas);
could be sufficient.
> @@ -549,8 +550,8 @@ static bool is_converted(char *sname)
> }
>
> /*
> - * Checks if symbol must be converted (conditions):
> - * not resolved, not already converted or isn't an exported symbol
> + * Checks if symbol must be or was already converted (conditions):
> + * not resolved or isn't an exported symbol
> */
> static bool must_convert(struct symbol *sym)
> {
> @@ -566,7 +567,7 @@ static bool must_convert(struct symbol *sym)
> if (strcmp(sym->name, ".TOC.") == 0)
> return false;
>
> - return (!(is_converted(sym->name) || is_exported(sym->name)));
> + return (!is_exported(sym->name));
> }
>
> /* Checks if a section is a klp rela section */
> @@ -640,7 +641,8 @@ int main(int argc, const char **argv)
> if (!must_convert(rela->sym))
> continue;
>
> - if (!find_missing_position(rela->sym, &sp)) {
> + if (!is_converted(rela->sym->name) &&
> + !find_missing_position(rela->sym, &sp)) {
> WARN("Unable to find missing symbol: %s",
> rela->sym->name);
> return -1;
Looks good.
Miroslav
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