oss-security - Re: Interesting behavior with struct initiailization

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Message-ID: <loom.20101205T175947-758@post.gmane.org>
Date: Sun, 5 Dec 2010 17:12:34 +0000 (UTC)
From: Bhadrinath <bitstrat@...il.com>
To: oss-security@...ts.openwall.com
Subject: Re: Interesting behavior with struct initiailization

One solution to ensure no padding bits are copied uninitialized,

    struct test{ int a; char b; int c;};


    unsigned char r[sizeof arg];

    struct test  arg = { .a = 1, .b = 2, .c = 3 };
    .
    .
    // Do all operations on arg and just before passing it to the function
    .
    .
    memset(r,0,sizeof r); // initialize everything to zero
    memcpy(r+offsetof(struct test,a),&arg.a,sizeof arg.a); 
    memcpy(r+offsetof(struct test,b),&arg.b,sizeof arg.b);
    memcpy(r+offsetof(struct test,c),&arg.c,sizeof arg.c);

    //now pass r to the function
    Copy_to_user(ptr, r, sizeof(r));

   Comments and ideas are welcome

With Regards
Bhadrinath

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