[PATCH] modpost: fix segfault with short symbol names

[PATCH] modpost: fix segfault with short symbol names

[Michal Marek]

memcmp() is wrong here, the symbol name can be shorter than KSYMTAB_PFX
or CRC_PFX.

Signed-off-by: Michal Marek <mmarek@suse.cz>
---
 scripts/mod/modpost.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 801a16a..065ee0b 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -515,7 +515,7 @@ static void handle_modversions(struct module *mod, struct elf_info *info,
 		break;
 	case SHN_ABS:
 		/* CRC'd symbol */
-		if (memcmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {
+		if (strncmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {
 			crc = (unsigned int) sym->st_value;
 			sym_update_crc(symname + strlen(CRC_PFX), mod, crc,
 					export);
@@ -559,7 +559,7 @@ static void handle_modversions(struct module *mod, struct elf_info *info,
 		break;
 	default:
 		/* All exported symbols */
-		if (memcmp(symname, KSYMTAB_PFX, strlen(KSYMTAB_PFX)) == 0) {
+		if (strncmp(symname, KSYMTAB_PFX, strlen(KSYMTAB_PFX)) == 0) {
 			sym_add_exported(symname + strlen(KSYMTAB_PFX), mod,
 					export);
 		}
-- 
1.6.5.3

Re: [PATCH] modpost: fix segfault with short symbol names

[Segher Boessenkool]

> memcmp() is wrong here, the symbol name can be shorter than  
> KSYMTAB_PFX
> or CRC_PFX.

> -		if (memcmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {
> +		if (strncmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {

This doesn't change anything.  In both cases the function will return 0
only if all strlen(CRC_PFX) chars match, and in both cases it can access
strlen(CRC_PFX) chars (strncmp() is allowed to access characters after
the first \0 just fine).


Segher

Re: [PATCH] modpost: fix segfault with short symbol names

[Michal Marek]

On Sat, Dec 12, 2009 at 04:26:20PM +0100, Segher Boessenkool wrote:
> >memcmp() is wrong here, the symbol name can be shorter than
> >KSYMTAB_PFX
> >or CRC_PFX.
> 
> >-		if (memcmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {
> >+		if (strncmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {
> 
> This doesn't change anything.  In both cases the function will return 0
> only if all strlen(CRC_PFX) chars match, and in both cases it can access
> strlen(CRC_PFX) chars (strncmp() is allowed to access characters after
> the first \0 just fine).

str(n)cmp is not allowed to access characters past the NUL byte.

Michal

Re: [PATCH] modpost: fix segfault with short symbol names

[Segher Boessenkool]

>>> memcmp() is wrong here, the symbol name can be shorter than
>>> KSYMTAB_PFX
>>> or CRC_PFX.
>>
>>> -		if (memcmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {
>>> +		if (strncmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {
>>
>> This doesn't change anything.  In both cases the function will  
>> return 0
>> only if all strlen(CRC_PFX) chars match, and in both cases it can  
>> access
>> strlen(CRC_PFX) chars (strncmp() is allowed to access characters  
>> after
>> the first \0 just fine).
>
> str(n)cmp is not allowed to access characters past the NUL byte.

You are wrong.  strncmp() _is_ allowed to do that.  So this patch
doesn't change anything.


Segher

Re: [PATCH] modpost: fix segfault with short symbol names

[Segher Boessenkool]

> On Sat, Dec 12, 2009 at 04:26:20PM +0100, Segher Boessenkool wrote:
>> >memcmp() is wrong here, the symbol name can be shorter than
>> >KSYMTAB_PFX
>> >or CRC_PFX.
>>
>> >-		if (memcmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {
>> >+		if (strncmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {
>>
>> This doesn't change anything.  In both cases the function will return 0
>> only if all strlen(CRC_PFX) chars match, and in both cases it can access
>> strlen(CRC_PFX) chars (strncmp() is allowed to access characters after
>> the first \0 just fine).
>
> str(n)cmp is not allowed to access characters past the NUL byte.

Let me apologise for the confusion.

strncmp() _can_ access characters past the first NUL byte; and
many implementations do so (they do accesses per 32-bit word,
for example).  But strncmp() behaves as-if it didn't compare
anything past the first null character, so for example causing
a page fault if the string ends just before the end of a memory
page isn't allowed; OTOH, the parameters to memcmp() are required
to point to two objects, each at least "len" bytes big.

So the patch fixes a real problem.  I'm sorry for being dense.

Acked-by: Segher Boessenkool <segher@kernel.crashing.org>


Segher

Re: [PATCH] modpost: fix segfault with short symbol names

[Michal Marek]

On 14.12.2009 14:33, Segher Boessenkool wrote:
> So the patch fixes a real problem.  I'm sorry for being dense.
> 
> Acked-by: Segher Boessenkool <segher@kernel.crashing.org>

Thanks :). Rusty, I sent the patch to you because you have some other
modpost stuff in the rr tree. But I can add it to the kbuild tree as
well if you want.

Michal

Re: [PATCH] modpost: fix segfault with short symbol names

[Rusty Russell]

On Sat, 12 Dec 2009 09:32:24 pm Michal Marek wrote:
> memcmp() is wrong here, the symbol name can be shorter than KSYMTAB_PFX
> or CRC_PFX.
> 
> Signed-off-by: Michal Marek <mmarek@suse.cz>

Thanks, applied.

Rusty.

Re: [PATCH] modpost: fix segfault with short symbol names

[Rusty Russell]

On Tue, 15 Dec 2009 12:03:12 am Segher Boessenkool wrote:
> > On Sat, Dec 12, 2009 at 04:26:20PM +0100, Segher Boessenkool wrote:
> >> >-		if (memcmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {
> >> >+		if (strncmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {
> >>
> >> This doesn't change anything.  In both cases the function will return 0
> >> only if all strlen(CRC_PFX) chars match, and in both cases it can access
> >> strlen(CRC_PFX) chars (strncmp() is allowed to access characters after
> >> the first \0 just fine).
> >
> > str(n)cmp is not allowed to access characters past the NUL byte.
> 
> Let me apologise for the confusion.

Mea culpas must be short, to avoid wasting even more time.  And they must be
at least as absolute as the incorrect assertion was.  This helps us lurkers
and archive readers flush the crap from their brains.

Your mails used _ for emphasis and "You are wrong.".  Ouch.  I suggest:

	"I am *wrong*.  Sorry Michal for the brain fart."

Also, please stop channeling Richard B. Johnson.
Rusty.