Tobias,
On 5/21/20 2:15 PM, Tobias Stoeckmann wrote:
> The function make_message illustrates how to use vsnprintf to determine
> the required amount of memory for a specific format and its arguments.
>
> If make_message is called with a format which will use exactly INT_MAX
> characters (excluding '\0'), then the size++ calculation will overflow
> the signed integer "size", which is an undefined behaviour in C.
>
> Since malloc and vsnprintf rightfully take a size_t argument, I decided
> to use a size_t variable for size calculation. Therefore, this patched
> code uses variables of the same data types as expected by function
> arguments.
>
> Proof of concept (tested on Linux/glibc amd64):
>
> int main() { make_message("%647s%2147483000s", "", ""); }
>
> If the code is compiled with address sanitizer (gcc -fsanitize=address)
> you can see the following line, assuming that a signed integer overflow
> simply leads to INT_MIN:
>
> ==3094==WARNING: AddressSanitizer failed to allocate 0xffffffff80000000 bytes
Thanks for the nicely documented patch. Applied.
Cheers,
Michael
> ---
> man3/printf.3 | 15 +++++++++------
> 1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/man3/printf.3 b/man3/printf.3
> index 50e136ba6..827d9cbae 100644
> --- a/man3/printf.3
> +++ b/man3/printf.3
> @@ -1132,29 +1132,32 @@ To allocate a sufficiently large string and print into it
> char *
> make_message(const char *fmt, ...)
> {
> - int size = 0;
> + int n = 0;
> + size_t size = 0;
> char *p = NULL;
> va_list ap;
>
> /* Determine required size */
>
> va_start(ap, fmt);
> - size = vsnprintf(p, size, fmt, ap);
> + n = vsnprintf(p, size, fmt, ap);
> va_end(ap);
>
> - if (size < 0)
> + if (n < 0)
> return NULL;
>
> - size++; /* For '\e0' */
> + /* One extra byte for '\e0' */
> +
> + size = (size_t) n + 1;
> p = malloc(size);
> if (p == NULL)
> return NULL;
>
> va_start(ap, fmt);
> - size = vsnprintf(p, size, fmt, ap);
> + n = vsnprintf(p, size, fmt, ap);
> va_end(ap);
>
> - if (size < 0) {
> + if (n < 0) {
> free(p);
> return NULL;
> }
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
