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

[Josef Möllers]

On 08.09.20 16:03, Amit anand wrote:
> Hi,
>
> On Tue, Sep 8, 2020 at 6:23 PM Ulrich Windl
> <Ulrich.Windl at rz.uni-regensburg.de
> <mailto:Ulrich.Windl at rz.uni-regensburg.de>> wrote:
>
>     Hi!
>
>     vbuf is initialized: It has some address on the stack, so "if
>     (value != vbuf)" is comparing adresses, if I got it right...
>
> vbuf having some address on stack and "if(value!=vbuf)" is comparing
> addresses doesn't necessarily implies that vbuf is initialized.
> I am still not clear about where vbuf is initialized.

It is initialized with the strscpy() call.

The comparison just compares the pointer in *value (+) with the ADDRESS
of "vbuf"!

(+) Please ignore my previous remark. I don't know what I had been
smoking at that time!

So if "value" does not point at "vbuf", it points at some other buffer,
but the data in that buffer should be in "vbuf", so the contents of the
buffer "value" points at is copied into the buffer named "vbuf"!

If "value" points at "vbuf", the call to "sd_device_get_sysattr_value"
would have  already put the data into "vbuf" and thus the strscpy()
would not be needed, as it would copy "vbuf" into itself.

HTH,

Josef

> Also, expecting some information regarding my queries in below mail.
>
> Thanks,
> Amit
>
>
>     >>> Amit anand <amit.table at gmail.com
>     <mailto:amit.table at gmail.com>> schrieb am 08.09.2020 um 13:56 in
>     Nachricht
>     <CAMkFpM5NFmN+pc_cztmMZW=j1vn2RYC16AgJ3LYL6RSgdqP1eA at mail.gmail.com
>     <mailto: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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