util/envlist: fix prefix-match in envlist_unsetenv() name lookup

envlist_unsetenv() looked up the entry to remove with
strncmp(entry->ev_var, env, strlen(env)). The comparison length is
the requested name's length, so any stored entry whose name *starts*
with that name compares equal. envlist_setenv() inserts at the head
of the list, so the first hit wins: with FOO=... stored first and
FOOBAR=... stored afterward, envlist_unsetenv("FOO") iterates from
the head, matches FOOBAR=... on the prefix, and drops it instead of
FOO=...

linux-user and bsd-user reach this code via the -U command-line
switch, so the bug is reachable from a normal qemu-user invocation.

envlist_setenv() used the same strncmp pattern but with
envname_len = (eq_sign - env + 1), so the '=' byte sat inside the
compared window and acted as an implicit boundary. setenv was
therefore not buggy -- but the safety lived in the byte layout of
ev_var rather than in the entry, so a future edit could easily
drift the two sites apart again.

Store the name length on each entry at insertion time and compare
with explicit length equality plus memcmp via a small helper. Use
the helper at both lookup sites so the boundary becomes a
structural property of the entry: envlist_unsetenv() stops
prefix-matching, and envlist_setenv()'s self-search no longer
depends on the '=' byte serving as a sentinel.

Fixes: 04a6dfebb6 ("linux-user: Add generic env variable handling")
Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20260520212628.479772-2-den@openvz.org
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

util/envlist.c

@@ -3,7 +3,8 @@
 #include "qemu/envlist.h"
 
 struct envlist_entry {
-    const char *ev_var;            /* actual env value */
+    const char *ev_var;            /* actual env value: "NAME=VALUE" */
+    size_t ev_name_len;            /* length of NAME (offset of '=') */
     QLIST_ENTRY(envlist_entry) ev_link;
 };
 
@@ -12,6 +13,13 @@ struct envlist {
     size_t el_count;                        /* number of entries */
 };
 
+static inline bool envlist_name_eq(const struct envlist_entry *entry,
+                                   const char *name, size_t name_len)
+{
+    return entry->ev_name_len == name_len &&
+           memcmp(entry->ev_var, name, name_len) == 0;
+}
+
 /*
  * Allocates new envlist and returns pointer to it.
  */
@@ -67,7 +75,7 @@ envlist_setenv(envlist_t *envlist, const char *env)
     /* find out first equals sign in given env */
     if ((eq_sign = strchr(env, '=')) == NULL)
         return (EINVAL);
-    envname_len = eq_sign - env + 1;
+    envname_len = eq_sign - env;
 
     /*
      * If there already exists variable with given name
@@ -76,8 +84,9 @@ envlist_setenv(envlist_t *envlist, const char *env)
      */
     for (entry = envlist->el_entries.lh_first; entry != NULL;
         entry = entry->ev_link.le_next) {
-        if (strncmp(entry->ev_var, env, envname_len) == 0)
+        if (envlist_name_eq(entry, env, envname_len)) {
             break;
+        }
     }
 
     if (entry != NULL) {
@@ -90,6 +99,7 @@ envlist_setenv(envlist_t *envlist, const char *env)
 
     entry = g_malloc(sizeof(*entry));
     entry->ev_var = g_strdup(env);
+    entry->ev_name_len = envname_len;
     QLIST_INSERT_HEAD(&envlist->el_entries, entry, ev_link);
 
     return (0);
@@ -119,8 +129,9 @@ envlist_unsetenv(envlist_t *envlist, const char *env)
     envname_len = strlen(env);
     for (entry = envlist->el_entries.lh_first; entry != NULL;
         entry = entry->ev_link.le_next) {
-        if (strncmp(entry->ev_var, env, envname_len) == 0)
+        if (envlist_name_eq(entry, env, envname_len)) {
             break;
+        }
     }
     if (entry != NULL) {
         QLIST_REMOVE(entry, ev_link);