[systemd-devel] Antw: [EXT] Initialization of "vbuf" in function "token_match_attr"

[Ulrich Windl]

Hi!

vbuf is initialized: It has some address on the stack, so "if (value != vbuf)" is comparing adresses, if I got it right...

>>> Amit anand <amit.table at gmail.com> schrieb am 08.09.2020 um 13:56 in Nachricht
<CAMkFpM5NFmN+pc_cztmMZW=j1vn2RYC16AgJ3LYL6RSgdqP1eA at mail.gmail.com>:
> Hi systemd-devel team,
> 
> I am trying to understand where "vbuf" is initialized in function
> "token_match_attr()" in file udev-rules.c
> 
> Below code snippet :
> static bool* token_match_attr*(UdevRuleToken *token, sd_device *dev,
> UdevEvent *event) {
>  *char* nbuf[UTIL_NAME_SIZE], *vbuf*[UTIL_NAME_SIZE];     // Here, vbuf is
> defined, however not initialized.
> *const char* *name, **value;        *// Here, value is declared to be of
> type const char *, however, not initialized.
> ...
> ... // some code
> ...
> switch (token->attr_subst_type) {   // *Event 1* : This evaluates to
> SUBST_TYPE_PLAIN
> ...
> case *SUBST_TYPE_PLAIN*:   if (sd_device_get_sysattr_value(dev, name,
> &value) < 0)   // *Event 2* :The if condition evaluates to false.
>                                                  return false;
>  break;
> ...// some code
> ...
> }
> 
>         if (token->attr_match_remove_trailing_whitespace) {      // *Event
> 4*: If condition evaluates to true
>                 if (value != vbuf) {      //  *Event 5* : vbuf and value
> are both declared but not initialized. Comparison is done without
> initializing "vbuf" and "value".
>                         strscpy(vbuf, sizeof(vbuf), value);
>                         value = vbuf;
>                 } // End of if.
> 
>                 delete_trailing_chars(vbuf, NULL);         // *Event 6*:
> trying to delete trailing chars from "vbuf" which is not initialzed. ??
>         } // End of outer if.
> ... // some code
> } /End of function : *token_match_attr()*
> 
> Below are my queries :
> 1. *Event 5 *above, we are comparing two resources ("value" and "vbuf")
> even before initializing them.
>     Are we doing this comparision to do decision making based on whether
> the pointer type variable "value" is pointing to "vbuf" or not.
> *Query*: Kindly confirm whether my understanding is correct.
> 
> 2. *Event 6* above, delete_trailing_chars(vbuf, NULL) is called outside  *if
> (value != vbuf).*
>     This implies there may occur a situation where:
>               step 1 ) *if (value != vbuf) *condition fails  --> step 2 )
> *vbuf* is not initialzed inside the *if(value != vbuf)  *--> step 3 )
> delete_trailing_chars(*vbuf*, NULL).
> Here, delete_trailing_chars() called for "*vbuf*" which is not necessarily
> initiazlied.
> *Query*: Kindly let me know if my understanding is correct or I am missing
> something.
> 
> Thanks,
> Amit