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Re: [PATCH] - improve sprintf buffer overflow detection (middle-end/49905)


On 07/04/2016 10:44 AM, Jakub Jelinek wrote:
On Mon, Jul 04, 2016 at 10:23:06AM -0600, Martin Sebor wrote:
1) Making use of -Wformat machinery in c-family/c-format.c.  This
    seemed preferable to duplicating some of the same code elsewhere
    (I initially started implementing it in expand_builtin in
    builtins.c).  It makes the implementation readily extensible
    to all the same formats as those already handled for -Wformat.
    One drawback is that unlike in expand_builtin, calls to these
    functions cannot readily be folded.  Another drawback pointed

folded?  You mean this -W option changes code generation?

No, it doesn't.  What I meant is that the same code, when added
in builtins.c instead, could readily be extended to fold into
strings expressions like

   sprintf (buf, "%i", 123);

I've commented in some PR a few years ago that I'm not convinced we want to
do it, or at least not without careful considerations, consider .rodata
size.  Say if the user has in 1000x different places
sprintf (buf, "foobarbaz %i", NNN); for various values of NNN, then such "optimization" would replace
a single string literal of length 13 bytes with 1000 string literals of 12-20 bytes.
Consider larger string literal, with %s and long additions and it might not
be a win even for 2 occurrences.

I agree that's something to consider.  But even if the call itself
weren't folded, the return value (i.e., the number of characters
computed by the checker) could be.

Martin


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