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  • how to fix brp and rpmlint warnings – today: I: Statement might be overflowing a buffer in strncat.

    Posted on May 28th, 2011 Dominique Leuenberger 2 comments

    Malcolm Lewis sent me a link to one of his failing packages, with the idea to bring the next post for this series (which has had a too long break already). So Malcolm: Thank you for the heads up and here we go with another hopefully helpful solution to one of brp error (brp actually stands for Build Root Policy)

    So, just like before, we have a look at the error, including the specific code parts raising the errors:

    I: Statement might be overflowing a buffer in strncat. Common mistake:
    BAD: strncat(buffer,charptr,sizeof(buffer)) is wrong, it takes the left over size as 3rd argument
    GOOD: strncat(buffer,charptr,sizeof(buffer)-strlen(buffer)-1)

    Ok, so the error is very helpful with even telling us what is a bad use of strncat and what a good use of it would look like. For completeness, let’s see what the compiler itself reports on this error:

    In function ‘strncat’,
    inlined from ‘gtkui_icmp_redir’ at ec_gtk_mitm.c:173:14:
    /usr/include/bits/string3.h:152:3: warning: call to __builtin___strncat_chk might overflow destination buffer [enabled by default]

    Let’s see what man 3 strncat tells us about the usage of strncat.


    char *strcat(char *dest, const char *src);
    char *strncat(char *dest, const char *src, size_t n);

    The strcat() function appends the src string to the dest string overwriting the ‘\0′ character at the end of dest, and then adds a terminating ‘\0′ character. The strings may not overlap, and the dest string must have enough space for the result.

    The strncat() function is similar, except that it will use at most n characters from src. Since the result is always terminated with ‘\0′, at most n+1 characters are written.

    The package suggested by Malcolm contains this code (snipped together):

    #define PARAMS_LEN 50
    static char params[PARAMS_LEN];

    strncat(params, gtk_entry_get_text(GTK_ENTRY(entry1)), PARAMS_LEN);
    strncat(params, "/", PARAMS_LEN);
    strncat(params, gtk_entry_get_text(GTK_ENTRY(entry2)), PARAMS_LEN);

    This is not an uncommon error at all. We create a buffer, 50 chars long, we concatenate text into the buffer and specify a max length of PARAMS_LEN. What is not taken into account here is that the string in params is getting longer, and we do not account for this when adding text to it. And additionally the leading \0 is ignored as well.

    Translating the BRP Result’s suggestion stating how we should write this code, we will result in this:
    #define PARAMS_LEN 50
    static char params[PARAMS_LEN];

    strncat(params, gtk_entry_get_text(GTK_ENTRY(entry1)), sizeof(params)-strlen(params)-1);
    strncat(params, "/", sizeof(params)-strlen(params)-1);
    strncat(params, gtk_entry_get_text(GTK_ENTRY(entry2)), sizeof(params)-strlen(params)-1);

    sizeof(params) is what we had there before, and equals to PARAMS_LEN. But we now also take into account that we already do have some characters stored in params and also account for a trailing NULL byte.

    And with this simple fix, the error vanished.

  • Enlightenment 1.0.0 for openSUSE

    Posted on February 1st, 2011 Dominique Leuenberger 5 comments

    It’s been a long time since I was in touch with Enlightenment, back in the young days I used it as my first alternative Desktop Environment on Windows, before actually daring to switch to Linux.

    Today I hear that version 1.0.0 of the core libraries have been released. Reason enough to actually look into this again. So 1 hour, many build failures and successes later, I have the entire core-stack built in my obs repository home:dimstar:Enlightenment. The window manager Enlightenment itself is not yet released as full release, but the latest snapshot can be found in my repo as well.

    But that’s it so far: all core libraries are built. Nothing more. I did not even have time to test them yet.

    If you feel adventurous, go, add the repository obs://home:dimstar:Enlightenment to your system (published for openSUSE 11.3 and Factory) and start playing with all of the things, starting with the window manager, which should generally pull in the rest.

    What should be required (as root)
    zypper ar obs://home:dimstar:Enlightenment Enlightenment
    zypper in enlightenment

    This will add my home repository to your catalog list and install enlightenment including dependencies. One logout later, you should see Enlightenment as a new session type in your *DM.

    Have a lot of fun and share your experiences… I will test it after I get some sleep.