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Date: Mon, 20 Apr 2020 15:01:41 -0400
From: Joe Lawrence <joe.lawrence@...hat.com>
To: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Jessica Yu <jeyu@...nel.org>
Subject: Re: [PATCH v2 2/9] livepatch: Apply vmlinux-specific KLP relocations
early
On Mon, Apr 20, 2020 at 01:25:16PM -0500, Josh Poimboeuf wrote:
> On Mon, Apr 20, 2020 at 01:57:51PM -0400, Joe Lawrence wrote:
> > On Fri, Apr 17, 2020 at 09:04:27AM -0500, Josh Poimboeuf wrote:
> > >
> > > [ ... snip ... ]
> > >
> > > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > > index 40cfac8156fd..5fda3afc0285 100644
> > > --- a/kernel/livepatch/core.c
> > > +++ b/kernel/livepatch/core.c
> > >
> > > [ ... snip ... ]
> > >
> > > +int klp_write_relocations(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
> > > + const char *shstrtab, const char *strtab,
> > > + unsigned int symndx, struct module *pmod,
> > > + const char *objname)
> > > {
> > > int i, cnt, ret = 0;
> > > - const char *objname, *secname;
> > > char sec_objname[MODULE_NAME_LEN];
> > > Elf_Shdr *sec;
> > >
> > > - if (WARN_ON(!klp_is_object_loaded(obj)))
> > > - return -EINVAL;
> > > -
> > > - objname = klp_is_module(obj) ? obj->name : "vmlinux";
> > > -
> > > /* For each klp relocation section */
> > > - for (i = 1; i < pmod->klp_info->hdr.e_shnum; i++) {
> > > - sec = pmod->klp_info->sechdrs + i;
> > > - secname = pmod->klp_info->secstrings + sec->sh_name;
> > > + for (i = 1; i < ehdr->e_shnum; i++) {
> > > + sec = sechdrs + i;
> >
> > Hi Josh, minor bug:
> >
> > Note the for loop through the section headers in
> > klp_write_relocations(), but its calling function ...
> >
> > > [ ... snip ... ]
> > >
> > > diff --git a/kernel/module.c b/kernel/module.c
> > > index 646f1e2330d2..d36ea8a8c3ec 100644
> > > --- a/kernel/module.c
> > > +++ b/kernel/module.c
> > > @@ -2334,11 +2334,12 @@ static int apply_relocations(struct module *mod, const struct load_info *info)
> > > if (!(info->sechdrs[infosec].sh_flags & SHF_ALLOC))
> > > continue;
> > >
> > > - /* Livepatch relocation sections are applied by livepatch */
> > > if (info->sechdrs[i].sh_flags & SHF_RELA_LIVEPATCH)
> > > - continue;
> > > -
> > > - if (info->sechdrs[i].sh_type == SHT_REL)
> > > + err = klp_write_relocations(info->hdr, info->sechdrs,
> > > + info->secstrings,
> > > + info->strtab,
> > > + info->index.sym, mod, NULL);
> > > + else if (info->sechdrs[i].sh_type == SHT_REL)
> > > err = apply_relocate(info->sechdrs, info->strtab,
> > > info->index.sym, i, mod);
> > > else if (info->sechdrs[i].sh_type == SHT_RELA)
> >
> > ... apply_relocations() is also iterating over the section headers (the
> > diff context doesn't show it here, but i is an incrementing index over
> > sechdrs[]).
> >
> > So if there is more than one KLP relocation section, we'll process them
> > multiple times. At least the x86 relocation code will detect this and
> > fail the module load with an invalid relocation (existing value not
> > zero).
>
> Ah, yes, good catch!
>
The same test case passed with a small modification to push the foreach
KLP section part to a kernel/livepatch/core.c local function and
exposing the klp_resolve_symbols() + apply_relocate_add() for a given
section to kernel/module.c. Something like following...
-- Joe
-->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8--
diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index 8b6886c2d5c7..516f285ccc82 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -232,9 +232,9 @@ void klp_shadow_free_all(unsigned long id, klp_shadow_dtor_t dtor);
struct klp_state *klp_get_state(struct klp_patch *patch, unsigned long id);
struct klp_state *klp_get_prev_state(unsigned long id);
-int klp_write_relocations(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
- const char *shstrtab, const char *strtab,
- unsigned int symindex, struct module *pmod,
+int klp_write_relocations(Elf_Shdr *sechdrs, const char *shstrtab,
+ const char *strtab, unsigned int symndx,
+ unsigned int relsec, struct module *pmod,
const char *objname);
/* Used to annotate symbol relocations in livepatches */
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index d2610f63e70b..d74fd7d10f16 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -285,14 +285,48 @@ static int klp_resolve_symbols(Elf64_Shdr *sechdrs, const char *strtab,
* the to-be-patched module to be loaded and patched sometime *after* the
* klp module is loaded.
*/
-int klp_write_relocations(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
- const char *shstrtab, const char *strtab,
- unsigned int symndx, struct module *pmod,
+
+int klp_write_relocations(Elf_Shdr *sechdrs, const char *shstrtab,
+ const char *strtab, unsigned int symndx,
+ unsigned int relsec, struct module *pmod,
const char *objname)
{
- int i, cnt, ret = 0;
char sec_objname[MODULE_NAME_LEN];
Elf_Shdr *sec;
+ int cnt, ret;
+
+ sec = sechdrs + relsec;
+
+ /*
+ * Format: .klp.rela.sec_objname.section_name
+ * See comment in klp_resolve_symbols() for an explanation
+ * of the selected field width value.
+ */
+ cnt = sscanf(shstrtab + sec->sh_name, KLP_RELA_PREFIX "%55[^.]",
+ sec_objname);
+ if (cnt != 1) {
+ pr_err("section %s has an incorrectly formatted name\n",
+ shstrtab + sec->sh_name);
+ return -EINVAL;
+ }
+
+ if (strcmp(objname ? objname : "vmlinux", sec_objname))
+ return 0;
+
+ ret = klp_resolve_symbols(sechdrs, strtab, symndx, sec, sec_objname);
+ if (ret)
+ return ret;
+
+ return apply_relocate_add(sechdrs, strtab, symndx, relsec, pmod);
+}
+
+static int klp_write_all_relocations(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
+ const char *shstrtab, const char *strtab,
+ unsigned int symndx, struct module *pmod,
+ const char *objname)
+{
+ int i, ret;
+ Elf_Shdr *sec;
/* For each klp relocation section */
for (i = 1; i < ehdr->e_shnum; i++) {
@@ -300,34 +334,13 @@ int klp_write_relocations(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
if (!(sec->sh_flags & SHF_RELA_LIVEPATCH))
continue;
- /*
- * Format: .klp.rela.sec_objname.section_name
- * See comment in klp_resolve_symbols() for an explanation
- * of the selected field width value.
- */
- cnt = sscanf(shstrtab + sec->sh_name, KLP_RELA_PREFIX "%55[^.]",
- sec_objname);
- if (cnt != 1) {
- pr_err("section %s has an incorrectly formatted name\n",
- shstrtab + sec->sh_name);
- ret = -EINVAL;
- break;
- }
-
- if (strcmp(objname ? objname : "vmlinux", sec_objname))
- continue;
-
- ret = klp_resolve_symbols(sechdrs, strtab, symndx, sec,
- sec_objname);
+ ret = klp_write_relocations(sechdrs, shstrtab, strtab, symndx,
+ i, pmod, objname);
if (ret)
- break;
-
- ret = apply_relocate_add(sechdrs, strtab, symndx, i, pmod);
- if (ret)
- break;
+ return ret;
}
- return ret;
+ return 0;
}
/*
@@ -773,11 +786,11 @@ static int klp_init_object_loaded(struct klp_patch *patch,
* written earlier during the initialization of the klp module
* itself.
*/
- ret = klp_write_relocations(&info->hdr, info->sechdrs,
- info->secstrings,
- patch->mod->core_kallsyms.strtab,
- info->symndx, patch->mod,
- obj->name);
+ ret = klp_write_all_relocations(&info->hdr, info->sechdrs,
+ info->secstrings,
+ patch->mod->core_kallsyms.strtab,
+ info->symndx, patch->mod,
+ obj->name);
if (ret)
return ret;
}
diff --git a/kernel/module.c b/kernel/module.c
index 86736e2ff73d..04e5f5d55eb4 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2322,10 +2322,11 @@ static int apply_relocations(struct module *mod, const struct load_info *info)
continue;
if (info->sechdrs[i].sh_flags & SHF_RELA_LIVEPATCH)
- err = klp_write_relocations(info->hdr, info->sechdrs,
+ err = klp_write_relocations(info->sechdrs,
info->secstrings,
info->strtab,
- info->index.sym, mod, NULL);
+ info->index.sym, i, mod,
+ NULL);
else if (info->sechdrs[i].sh_type == SHT_REL)
err = apply_relocate(info->sechdrs, info->strtab,
info->index.sym, i, mod);
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