Re: [PATCH] git name-rev writes beyond the end of malloc() with large generations

> When using git name-rev on my kernel tree I triggered a malloc()
> corruption warning from glibc.
>
> apw@pinky$ git log --pretty=one $N/base.. | git name-rev --stdin
> *** glibc detected *** malloc(): memory corruption: 0x0bff8950 ***
> Aborted
>
> This comes from name_rev() which is building the name of the revision
> in a malloc'd string, which it sprintf's into:
>
> 	char *new_name = xmalloc(len + 8);
> 	[...]
> 		sprintf(new_name, "%.*s~%d^%d", len, tip_name,
> 				generation, parent_number);
>
> This allocation is only sufficient if the generation number is
> less than 5 digits, in my case generation was 13432.  In reality
> parent_number can be up to 16 so that also can require two digits,
> reducing us to 3 digits before we are at risk of blowing this
> allocation.
>
> This patch introduces a decimal_length() which approximates the
> number of digits a type may hold, it produces the following:
> ...

> diff --git a/git-compat-util.h b/git-compat-util.h
> index c08688c..25b8274 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -19,6 +19,9 @@
>  #define TYPEOF(x)
>  #endif
>  
> +/* Approximation of the length of the decimal representation of this type. */
> +#define decimal_length(x)	((int)(sizeof(x) * 2.56 + 0.5) + 1)
> +
>  #define MSB(x, bits) ((x) & TYPEOF(x)(~0ULL << (sizeof(x) * 8 - (bits))))
>  
>  #if !defined(__APPLE__) && !defined(__FreeBSD__)