[PATCH 2/2] ARM: module: Fix function kallsyms on Thumb-2

[Vincent Whitchurch <vincent.whitchurch@axis.com>]

Thumb-2 functions have the lowest bit set in the symbol value in the
symtab.  When kallsyms are generated for the vmlinux, the kallsyms are
generated from the output of nm, and nm clears the lowest bit.

 $ arm-linux-gnueabihf-readelf -a vmlinux | grep show_interrupts
  95947: 8015dc89   686 FUNC    GLOBAL DEFAULT    2 show_interrupts
 $ arm-linux-gnueabihf-nm vmlinux | grep show_interrupts
 8015dc88 T show_interrupts
 $ cat /proc/kallsyms | grep show_interrupts
 8015dc88 T show_interrupts

However, for modules, the kallsyms uses the values in the symbol table
without modification, so for functions in modules, the lowest bit is set
in kallsyms.

 $ arm-linux-gnueabihf-readelf -a drivers/net/tun.ko | grep tun_get_socket
    333: 00002d4d    36 FUNC    GLOBAL DEFAULT    1 tun_get_socket
 $ arm-linux-gnueabihf-nm drivers/net/tun.ko | grep tun_get_socket
 00002d4c T tun_get_socket
 $ cat /proc/kallsyms | grep tun_get_socket
 7f802d4d t tun_get_socket      [tun]

Because of this, the symbol+offset of the crashing instruction shown in
oopses is incorrect when the crash is in a module.  For example, given a
tun_get_socket which starts like this,

 00002d4c <tun_get_socket>:
     2d4c:       6943            ldr     r3, [r0, #20]
     2d4e:       4a07            ldr     r2, [pc, #28]
     2d50:       4293            cmp     r3, r2

a crash when tun_get_socket is called with NULL results in:

 PC is at tun_xdp+0xa3/0xa4 [tun]
 pc : [<7f802d4c>]

As can be seen, the "PC is at" line reports the wrong symbol name, and
the symbol+offset will point to the wrong source line if it is passed to
gdb.

To solve this, add a way for archs to fixup the reading of these module
kallsyms values, and use that to clear the lowest bit for function
symbols on Thumb-2.

After the fix:

 # cat /proc/kallsyms | grep tun_get_socket
 7f802d4c t tun_get_socket       [tun]

 PC is at tun_get_socket+0x0/0x24 [tun]
 pc : [<7f802d4c>]

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
---
v4: Split out st_value overwrite change.  Add HAVE* macro to avoid function call.
v3: Do not overwrite st_value
v2: Fix build warning with !MODULES

 arch/arm/include/asm/module.h | 11 +++++++++++
 include/linux/module.h        |  7 +++++++
 kernel/module.c               | 25 ++++++++++++++-----------
 3 files changed, 32 insertions(+), 11 deletions(-)

diff --git a/arch/arm/include/asm/module.h b/arch/arm/include/asm/module.h
index 9e81b7c498d8..e2ccec651e71 100644
--- a/arch/arm/include/asm/module.h
+++ b/arch/arm/include/asm/module.h
@@ -61,4 +61,15 @@ u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val);
 	MODULE_ARCH_VERMAGIC_ARMTHUMB \
 	MODULE_ARCH_VERMAGIC_P2V
 
+#ifdef CONFIG_THUMB2_KERNEL
+#define HAVE_ARCH_MODULE_KALLSYMS_SYMBOL_VALUE
+static inline unsigned long module_kallsyms_symbol_value(Elf_Sym *sym)
+{
+	if (ELF_ST_TYPE(sym->st_info) == STT_FUNC)
+		return sym->st_value & ~1;
+
+	return sym->st_value;
+}
+#endif
+
 #endif /* _ASM_ARM_MODULE_H */
diff --git a/include/linux/module.h b/include/linux/module.h
index fce6b4335e36..cfc55f358800 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -486,6 +486,13 @@ struct module {
 #define MODULE_ARCH_INIT {}
 #endif
 
+#ifndef HAVE_ARCH_MODULE_KALLSYMS_SYMBOL_VALUE
+static inline unsigned long module_kallsyms_symbol_value(Elf_Sym *sym)
+{
+	return sym->st_value;
+}
+#endif
+
 extern struct mutex module_mutex;
 
 /* FIXME: It'd be nice to isolate modules during init, too, so they
diff --git a/kernel/module.c b/kernel/module.c
index 3d86a38b580c..a36d7915ed2b 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3934,6 +3934,9 @@ static const char *get_ksymbol(struct module *mod,
 	/* Scan for closest preceding symbol, and next symbol. (ELF
 	   starts real symbols at 1). */
 	for (i = 1; i < kallsyms->num_symtab; i++) {
+		unsigned long thisval = module_kallsyms_symbol_value(&kallsyms->symtab[i]);
+		unsigned long bestval = module_kallsyms_symbol_value(&kallsyms->symtab[best]);
+
 		if (kallsyms->symtab[i].st_shndx == SHN_UNDEF)
 			continue;
 
@@ -3943,21 +3946,21 @@ static const char *get_ksymbol(struct module *mod,
 		    || is_arm_mapping_symbol(symname(kallsyms, i)))
 			continue;
 
-		if (kallsyms->symtab[i].st_value <= addr
-		    && kallsyms->symtab[i].st_value > kallsyms->symtab[best].st_value)
+		if (thisval <= addr
+		    && thisval > bestval)
 			best = i;
-		if (kallsyms->symtab[i].st_value > addr
-		    && kallsyms->symtab[i].st_value < nextval)
-			nextval = kallsyms->symtab[i].st_value;
+		if (thisval > addr
+		    && thisval < nextval)
+			nextval = thisval;
 	}
 
 	if (!best)
 		return NULL;
 
 	if (size)
-		*size = nextval - kallsyms->symtab[best].st_value;
+		*size = nextval - module_kallsyms_symbol_value(&kallsyms->symtab[best]);
 	if (offset)
-		*offset = addr - kallsyms->symtab[best].st_value;
+		*offset = addr - module_kallsyms_symbol_value(&kallsyms->symtab[best]);
 	return symname(kallsyms, best);
 }
 
@@ -4060,7 +4063,7 @@ int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
 			continue;
 		kallsyms = rcu_dereference_sched(mod->kallsyms);
 		if (symnum < kallsyms->num_symtab) {
-			*value = kallsyms->symtab[symnum].st_value;
+			*value = module_kallsyms_symbol_value(&kallsyms->symtab[symnum]);
 			*type = kallsyms->symtab[symnum].st_size;
 			strlcpy(name, symname(kallsyms, symnum), KSYM_NAME_LEN);
 			strlcpy(module_name, mod->name, MODULE_NAME_LEN);
@@ -4082,7 +4085,7 @@ static unsigned long mod_find_symname(struct module *mod, const char *name)
 	for (i = 0; i < kallsyms->num_symtab; i++)
 		if (strcmp(name, symname(kallsyms, i)) == 0 &&
 		    kallsyms->symtab[i].st_shndx != SHN_UNDEF)
-			return kallsyms->symtab[i].st_value;
+			return module_kallsyms_symbol_value(&kallsyms->symtab[i]);
 	return 0;
 }
 
@@ -4131,8 +4134,8 @@ int module_kallsyms_on_each_symbol(int (*fn)(void *, const char *,
 			if (kallsyms->symtab[i].st_shndx == SHN_UNDEF)
 				continue;
 
-			ret = fn(data, symname(kallsyms, i),
-				 mod, kallsyms->symtab[i].st_value);
+			ret = fn(data, symname(kallsyms, i), mod,
+				 module_kallsyms_symbol_value(&kallsyms->symtab[i]));
 			if (ret != 0)
 				return ret;
 		}
-- 
2.11.0