Re: safe strcpy()?

From: Ed Carp (erc@pobox.com)
Date: 01/28/03

  • Next message: Michal Zalewski: "Re: safe strcpy()?" 
    Date: Tue, 28 Jan 2003 03:44:24 -0600 (CST)
From: Ed Carp <erc@pobox.com>
To: Michal Zalewski <lcamtuf@coredump.cx>

    On Tue, 28 Jan 2003, Michal Zalewski wrote:

    > There are several interesting ways to prevent the problem without making
    > major changes to the code, however. One of better ideas I've seen is to
    > register buffer sizes when buffers are created. It takes few more lines
    > when you create an object, but this is the only effort you need to make.
    > Buffers are later deregistered from your own free(), for heap memory - and
    > your own function epilogue, for stack. With some help from the compiler
    > and linker, two last steps involve no changes to your existing code. If
    > your code uses mapped memory, you might also want to cover munmap and
    > such.
    >
    > You just call register_buf() whenever you create an array, a structure or
    > such. Its address, length and element size would be stored, so that you
    > can query for a buffer at any time, and perhaps decide, based on element
    > size, if its suitable for the operation you are about to perform (so that
    > when you have a number of buffers inside an array, and you only added the
    > array to implement index range checking, but forgot to add single buffers,
    > your code would not overwrite other elements when modifying one, but
    > rather bail out because of element size mismatch).
    >
    > The approach is not perfect, but can be quite helpful.

    I wasn't able to find such a function - do you have an example?

    Most of the buffers we use are fixed-size, to side-step problems with
    malloc() and free(), and so we've been able to partially get around this
    problem by writing strcpy() as a macro - for example:

    char buf[512];

    our_strcpy(buf, source);

    if our_strcpy() is written as a macro, then sizeof(buf) will return 512,
    and so we can do bounds checking. The problem comes in when someone does
    something like:

    ptr = buf;
    our_strcpy(ptr, source);

    How can one determine the size of the buffer being pointed to?
    sizeof(ptr) returns 4 :( Technically, that's correct, but that's not what
    I meant ;) 

    -- 
Ed Carp, N7EKG          http://www.pobox.com/~erc               214/986-5870
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