Re: [yocto-patches] [pseudo] [PATCH 1/7] pseudo.h: Avoid accessing unallocated memory

On 7/1/26 11:47 AM, Richard Purdie via lists.yoctoproject.org wrote:
> On Wed, 2026-07-01 at 11:40 -0500, Mark Hatle wrote:
>> On 7/1/26 8:13 AM, Richard Purdie wrote:
>>> We can call STARTSWITH in cases where the item being searched for is longer
>>> than the string itself. Switch from memcmp to strncmp to avoid accessing
>>> unassigned memory.
>>>
>>> Signed-off-by: Richard Purdie <[email protected]>
>>> ---
>>>    pseudo.h | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/pseudo.h b/pseudo.h
>>> index b6c13f2..1152c19 100644
>>> --- a/pseudo.h
>>> +++ b/pseudo.h
>>> @@ -99,7 +99,7 @@ extern char *pseudo_version;
>>>     #define PSEUDO_LIBDIR "lib"
>>>    #endif
>>>    
>>> -#define STARTSWITH(x, y) (!memcmp((x), (y), sizeof(y) - 1))
>>> +#define STARTSWITH(x, y) (strncmp(y, x, strlen(y)) == 0)
>>
>> I've not looked at the users of STARTSWITH.  If they are all strings this is
>> fine.  I believe memcmp was originally used because it could be items with nulls
>> in it.  The bug may be that 'sizeof' was not calculated properly (specifically
>> checking both X and Y, only y.. which can lead that the issue.)
> 
> It is only used by pseudo_util.c against envp:
> 
> pseudo_util.c:		if (STARTSWITH(envp[i], PRELINK_LIBRARIES "=")) {
> pseudo_util.c:		if (STARTSWITH(envp[i], PRELINK_LIBRARIES "=")) {
> pseudo_util.c:		if (STARTSWITH(envp[i], PRELINK_PATH "=")) {
> pseudo_util.c:		if (STARTSWITH(envp[i], PRELINK_LIBRARIES "=")) continue;
> pseudo_util.c:		if (STARTSWITH(envp[i], PRELINK_PATH "=")) continue;
> 
> i.e. always on strings.
> 
> The bug is that y can be longer than x and the memcmp check doesn't
> check for that. I could have put more logic in there but we may as well
> just use strncmp.

Agreed, if we're limiting to strings then better to use strncmp.

--Mark

> Cheers,
> 
> Richard
> 
> 
> -=-=-=-=-=-=-=-=-=-=-=-
> Links: You receive all messages sent to this group.
> View/Reply Online (#4351): https://lists.yoctoproject.org/g/yocto-patches/message/4351
> Mute This Topic: https://lists.yoctoproject.org/mt/120064771/3616948
> Group Owner: [email protected]
> Unsubscribe: https://lists.yoctoproject.org/g/yocto-patches/leave/13201099/3616948/947757854/xyzzy [[email protected]]
> -=-=-=-=-=-=-=-=-=-=-=-
> 
>