[Slirp] [PATCH] Remove alloca() call in get_dns_addr_resolv_conf()

[Philippe Mathieu-Daudé]

The ALLOCA(3) man-page mentions its "use is discouraged".

For now get_dns_addr_resolv_conf() is called with pointer to
a in_addr/in6_addr structure, and its size. Declare a union
of these structures on the stack, able to hold both of them.
This allows us to remove the alloca() call, keeping the buffer
on the stack.

Add an assertion in the unlikely case another inet address
is handled by this function.

Signed-off-by: Philippe Mathieu-Daudé <philmd at redhat.com>
---
Sorry no pull-req, I don't have my 2FA token with me.
---
 src/slirp.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/src/slirp.c b/src/slirp.c
index ee4ce36..5d60cb5 100644
--- a/src/slirp.c
+++ b/src/slirp.c
@@ -249,9 +249,13 @@ static int get_dns_addr_resolv_conf(int af, void *pdns_addr, void *cached_addr,
     char buff2[257];
     FILE *f;
     int found = 0;
-    void *tmp_addr = alloca(addrlen);
+    union {
+        struct in_addr dns_addr;
+        struct in6_addr dns6_addr;
+    } tmp_addr;
     unsigned if_index;
 
+    assert(sizeof(tmp_addr) >= addrlen);
     f = fopen("/etc/resolv.conf", "r");
     if (!f)
         return -1;
@@ -267,13 +271,13 @@ static int get_dns_addr_resolv_conf(int af, void *pdns_addr, void *cached_addr,
                 if_index = 0;
             }
 
-            if (!inet_pton(af, buff2, tmp_addr)) {
+            if (!inet_pton(af, buff2, &tmp_addr)) {
                 continue;
             }
             /* If it's the first one, set it to dns_addr */
             if (!found) {
-                memcpy(pdns_addr, tmp_addr, addrlen);
-                memcpy(cached_addr, tmp_addr, addrlen);
+                memcpy(pdns_addr, &tmp_addr, addrlen);
+                memcpy(cached_addr, &tmp_addr, addrlen);
                 if (scope_id) {
                     *scope_id = if_index;
                 }
@@ -285,7 +289,7 @@ static int get_dns_addr_resolv_conf(int af, void *pdns_addr, void *cached_addr,
                 break;
             } else if (slirp_debug & DBG_MISC) {
                 char s[INET6_ADDRSTRLEN];
-                const char *res = inet_ntop(af, tmp_addr, s, sizeof(s));
+                const char *res = inet_ntop(af, &tmp_addr, s, sizeof(s));
                 if (!res) {
                     res = "  (string conversion error)";
                 }
-- 
2.26.3