[PATCH] mem-pool: use st_add() in mem_pool_strvfmt()

["René Scharfe" <l.s.r@web.de>]

If len is INT_MAX in mem_pool_strvfmt(), then len + 1 overflows.
Casting it to size_t would prevent that.  Use st_add() to go a step
further and make the addition *obviously* safe.  The compiler can
optimize the check away on platforms where SIZE_MAX > INT_MAX, i.e.
basically everywhere.

Signed-off-by: René Scharfe <l.s.r@web.de>
---
 mem-pool.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mem-pool.c b/mem-pool.c
index 2078c22b09..3065b12b23 100644
--- a/mem-pool.c
+++ b/mem-pool.c
@@ -115,6 +115,7 @@ static char *mem_pool_strvfmt(struct mem_pool *pool, const char *fmt,
 	size_t available = block ? block->end - block->next_free : 0;
 	va_list cp;
 	int len, len2;
+	size_t size;
 	char *ret;

 	va_copy(cp, ap);
@@ -123,13 +124,14 @@ static char *mem_pool_strvfmt(struct mem_pool *pool, const char *fmt,
 	if (len < 0)
 		BUG("your vsnprintf is broken (returned %d)", len);

-	ret = mem_pool_alloc(pool, len + 1);  /* 1 for NUL */
+	size = st_add(len, 1); /* 1 for NUL */
+	ret = mem_pool_alloc(pool, size);

 	/* Shortcut; relies on mem_pool_alloc() not touching buffer contents. */
 	if (ret == next_free)
 		return ret;

-	len2 = vsnprintf(ret, len + 1, fmt, ap);
+	len2 = vsnprintf(ret, size, fmt, ap);
 	if (len2 != len)
 		BUG("your vsnprintf is broken (returns inconsistent lengths)");
 	return ret;
--
2.44.0