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

[Josef Möllers]

On 08.09.20 14:53, Ulrich Windl wrote:
> Hi!
>
> vbuf is initialized: It has some address on the stack, so "if (value != vbuf)" is comparing adresses, if I got it right...

You got it right, but I stumble across this (in essence):

char vbuf[UTIL_NAME_SIZE];
char **value;
if (value != vbuf)
    strscpy(vbuf, sizeof(vbuf), value);

"vbuf" has a type of "pointer to char" and "values" has type "pointer to
pointer to char". The logic dictates that "value" should also be "just"
a "char *value".

But I guess that's a copy-paste-error.

Josef

>
>>>> 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
>
>
>
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