Re: livepatch: reuse module loader code to write relocations

+++ Petr Mladek [09/02/16 15:01 +0100]:

On Wed 2016-02-03 20:11:09, Jessica Yu wrote:

quoted

Reuse module loader code to write relocations, thereby eliminating the need
for architecture specific relocation code in livepatch. Specifically, reuse
the apply_relocate_add() function in the module loader to write relocations
instead of duplicating functionality in livepatch's arch-dependent
klp_write_module_reloc() function.

In order to accomplish this, livepatch modules manage their own relocation
sections (marked with the SHF_RELA_LIVEPATCH section flag) and
livepatch-specific symbols (marked with SHN_LIVEPATCH symbol section
index). To apply livepatch relocation sections, livepatch symbols
referenced by relocs are resolved and then apply_relocate_add() is called
to apply those relocations.

In addition, remove x86 livepatch relocation code and the s390
klp_write_module_reloc() function stub. They are no longer needed since
relocation work has been offloaded to module loader.

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 7aa975d..c1fe57c 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c

@@ -28,6 +28,9 @@
 #include <linux/list.h>
 #include <linux/kallsyms.h>
 #include <linux/livepatch.h>
+#include <linux/elf.h>
+#include <linux/string.h>
+#include <linux/moduleloader.h>
 #include <asm/cacheflush.h>

 /**

@@ -87,6 +90,166 @@ static bool klp_is_object_loaded(struct klp_object *obj)
 	return !obj->name || obj->mod;
 }

+/*
+ * Check if a livepatch symbol is formatted properly.
+ *
+ * See Documentation/livepatch/module-elf-format.txt for a
+ * detailed outline of requirements.
+ */
+static int klp_check_symbol_format(struct module *pmod, Elf_Sym *sym)
+{
+	size_t len;
+	char *s, *objname, *symname;
+
+	if (sym->st_shndx != SHN_LIVEPATCH)
+		return -EINVAL;
+
+	/*
+	 * Livepatch symbol names must follow this format:
+	 * .klp.sym.objname.symbol_name,sympos
+	 */
+	s = pmod->strtab + sym->st_name;
+	/* [.klp.sym.]objname.symbol_name,sympos */
+	if (!s || strncmp(s, KLP_SYM_PREFIX, KLP_SYM_PREFIX_LEN))
+		return -EINVAL;
+
+	/* .klp.sym.[objname].symbol_name,sympos */
+	objname = s + KLP_SYM_PREFIX_LEN;
+	len = strcspn(objname, ".");
+	if (!(len > 0))
+		return -EINVAL;
+
+	/* .klp.sym.objname.symbol_name,[sympos] */
+	if (!strchr(s, ','))
+		return -EINVAL;
+
+	/* .klp.sym.objname.[symbol_name],sympos */
+	symname = objname + len + 1;
+	len = strcspn(symname, ",");
+	if (!(len > 0))
+		return -EINVAL;
+
+	return 0;
+}
+
+/*
+ * Check if a livepatch relocation section is formatted properly.
+ *
+ * See Documentation/livepatch/module-elf-format.txt for a
+ * detailed outline of requirements.
+ */
+static int klp_check_relasec_format(struct module *pmod, Elf_Shdr *relasec)
+{
+	char *secname;
+	size_t len;
+
+	secname = pmod->klp_info->secstrings + relasec->sh_name;
+	/* [.klp.rela.]objname.section_name */
+	if (!secname || strncmp(secname, KLP_RELASEC_PREFIX,
+				KLP_RELASEC_PREFIX_LEN))
+		return -EINVAL;
+
+	/* .klp.rela.[objname].section_name */
+	len = strcspn(secname + KLP_RELASEC_PREFIX_LEN, ".");
+	if (!(len > 0))
+		return -EINVAL;
+
+	return 0;
+}
+
+/*
+ * Check if obj->name matches the objname encoded in the rela
+ * section name (.klp.rela.[objname].section_name)
+ *
+ * Must pass klp_check_relasec_format() before calling this.
+ */
+static bool klp_relasec_matches_object(struct module *pmod, Elf_Shdr *relasec,
+				       struct klp_object *obj)
+{
+	size_t len;
+	const char *obj_objname, *sec_objname, *secname;
+
+	secname = pmod->klp_info->secstrings + relasec->sh_name;
+	/* .klp.rela.[objname].section_name */
+	sec_objname = secname + KLP_RELASEC_PREFIX_LEN;
+	obj_objname = klp_is_module(obj) ? obj->name : "vmlinux";
+
+	/* Get length of the objname encoded in the section name */
+	len = strcspn(sec_objname, ".");
+
+	if (strlen(obj_objname) != len)
+		return false;
+
+	return strncmp(sec_objname, obj_objname, len) ? false : true;
+}
+
+/*
+ * klp_get_* helper functions
+ *
+ * klp_get_* functions extract different components of the name
+ * of a livepatch symbol. The full symbol name from the strtab
+ * is passed in as parameter @s, and @result is filled in with
+ * the extracted component.
+ *
+ * These functions assume a correctly formatted symbol and the
+ * klp_check_symbol_format() test *must* pass before calling any
+ * of these functions.
+ */
+
+/* .klp.sym.[objname].symbol_name,sympos */
+static int klp_get_sym_objname(char *s, char **result)
+{
+	size_t len;
+	char *objname, *objname_start;
+
+	/* .klp.sym.[objname].symbol_name,sympos */
+	objname_start = s + KLP_SYM_PREFIX_LEN;
+	len = strcspn(objname_start, ".");
+	objname = kstrndup(objname_start, len, GFP_KERNEL);
+	if (objname == NULL)
+		return -ENOMEM;
+
+	/* klp_find_object_symbol() treats NULL as vmlinux */
+	if (!strcmp(objname, "vmlinux")) {
+		*result = NULL;
+		kfree(objname);
+	} else
+		*result = objname;
+
+	return 0;
+}
+
+/* .klp.sym.objname.[symbol_name],sympos */
+static int klp_get_symbol_name(char *s, char **result)
+{
+	size_t len;
+	char *objname, *symname;
+
+	/* .klp.sym.[objname].symbol_name,sympos */
+	objname = s + KLP_SYM_PREFIX_LEN;
+	len = strcspn(objname, ".");
+
+	/* .klp.sym.objname.[symbol_name],sympos */
+	symname = objname + len + 1;
+	len = strcspn(symname, ",");
+
+	*result = kstrndup(symname, len, GFP_KERNEL);
+	if (*result == NULL)
+		return -ENOMEM;
+
+	return 0;
+}
+
+/* .klp.sym.objname.symbol_name,[sympos] */
+static int klp_get_sympos(char *s, unsigned long *result)
+{
+	char *sympos;
+
+	/* .klp.sym.symbol_name,[sympos] */
+	sympos = strchr(s, ',') + 1;
+	return kstrtol(sympos, 10, result);
+}

The usage of the helper function is nicely strightforward.
Also I like a lot all the comments with the [] brackets
that highlight what each check or search is for.

But I think that there is a lot of duplicated code in
the check_ and in the get_ functions. Also there is
a lot of strdup/free games.

I agree, I do not really like the repetition and all
the strdup/free's myself.

The length of the elemets is limited by definition.
The functions are called under klp_mutex.

Therefore it might be easier to maintain a two parse
function that would fill static buffers and return
error in case of wrong value.

I mean something like:

static char mod_name[MODULE_NAME_LEN + 1];
static char symbol_name[KSYM_SYMBOL_LEN + 1];

static int
klp_parse_symbol_format(struct module *pmod, Elf_Sym *sym,
		char *mod_name, char *symbol_name,
		int *sympos)
{
char *substr;


/*
 * Livepatch symbol names must follow this format:
 * .klp.sym.objname.symbol_name,sympos
 */
s = pmod->strtab + sym->st_name;
/* [.klp.sym.]objname.symbol_name,sympos */
if (!s || strncmp(s, KLP_SYM_PREFIX, KLP_SYM_PREFIX_LEN))
	return -EINVAL;

/* .klp.sym.[objname].symbol_name,sympos */
substr = s + KLP_SYM_PREFIX_LEN;
len = strcspn(substr, ".");
if (!len || len > MODULE_NAME_LEN)
	return -EINVAL;
strncpy(mod_name, substr, len);
mod_name[len] = '\0';

/* .klp.sym.objname.[symbol_name],sympos */
substr = substr + len + 1;
len = strcspn(substr, ",");
if (!len || len > KSYM_SYMBOL_LEN)
	return -EINVAL;
strncpy(symbol_name, substr, len);
mod_name[len] = '\0';

/* .klp.sym.objname.symbol_name,[sympos] */
substr = substr + len + 1;
len = strlen(substr);
if (!len)
	return -EINVAL;

return kstrtol(substr, 10, sympos);
}

How does that sound, please?

I like this idea, it is a lot cleaner and more concise
than using all those helper functions. :-)

Thanks,
Jessica