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[PATCH] linux-user: NETLINK_LIST_MEMBERSHIPS: Allow bad ptr if its lengt

From: Frédéric Fortier
Subject: [PATCH] linux-user: NETLINK_LIST_MEMBERSHIPS: Allow bad ptr if its length is 0
Date: Sun, 28 Mar 2021 14:01:35 -0400

getsockopt(fd, SOL_NETLINK, NETLINK_LIST_MEMBERSHIPS, *optval, *optlen)
syscall allows optval to be NULL/invalid if optlen points to a size of
zero. This allows userspace to query the length of the array they should
use to get the full membership list before allocating memory for said
list, then re-calling getsockopt with proper optval/optlen arguments.

Notable users of this pattern include systemd-networkd, which in the
(albeit old) version 237 tested, cannot start without this fix.

Signed-off-by: Frédéric Fortier <frf@ghgsat.com>
 linux-user/syscall.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 1e508576c7..9b7556a9a2 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -3025,7 +3025,7 @@ get_timeout:
                 return -TARGET_EINVAL;
             results = lock_user(VERIFY_WRITE, optval_addr, len, 1);
-            if (!results) {
+            if (!results && len > 0) {
                 return -TARGET_EFAULT;
             lv = len;

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