[PATCH 7/7] block/ssh.c: Don't double-check that characters are hex digits

[Peter Maydell]

In compare_fingerprint() we effectively check whether the characters
in the fingerprint are valid hex digits twice: first we do so with
qemu_isxdigit(), but then the hex2decimal() function also has a code
path where it effectively detects an invalid digit and returns -1.
This causes Coverity to complain because it thinks that we might use
that -1 value in an expression where it would be an integer overflow.

Avoid the double-check of hex digit validity by testing the return
values from hex2decimal() rather than doing separate calls to
qemu_isxdigit().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
Could alternatively have put a g_assert_non_reached() in
hex2decimal(), but this seemed better to me.
---
 block/ssh.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/block/ssh.c b/block/ssh.c
index 27d582e0e3d..510dd208aba 100644
--- a/block/ssh.c
+++ b/block/ssh.c
@@ -376,13 +376,15 @@ static int compare_fingerprint(const unsigned char 
*fingerprint, size_t len,
     unsigned c;
 
     while (len > 0) {
+        unsigned c0, c1;
         while (*host_key_check == ':')
             host_key_check++;
-        if (!qemu_isxdigit(host_key_check[0]) ||
-            !qemu_isxdigit(host_key_check[1]))
+        c0 = hex2decimal(host_key_check[0]);
+        c1 = hex2decimal(host_key_check[1]);
+        if (c0 > 0xf || c1 > 0xf) {
             return 1;
-        c = hex2decimal(host_key_check[0]) * 16 +
-            hex2decimal(host_key_check[1]);
+        }
+        c = c0 * 16 + c1;
         if (c - *fingerprint != 0)
             return c - *fingerprint;
         fingerprint++;
-- 
2.34.1