The UML stub takes the address of CMSG_DATA(fd_msg):
fd_map = (void *)&CMSG_DATA(fd_msg);
CMSG_DATA() is specified by POSIX to return unsigned char *. Taking
its address is semantically wrong -- the intent is to get a pointer
to the control message data, which is exactly what CMSG_DATA()
already returns.
This happens to compile with glibc because glibc's primary
CMSG_DATA definition accesses a flexible array member:
#define CMSG_DATA(cmsg) ((cmsg)->__cmsg_data)
An array lvalue can have its address taken, and &array yields the
same address as array. However, glibc also has an alternative
definition that uses pointer arithmetic (returning an rvalue), and
musl's definition always uses pointer arithmetic:
/* musl */
#define CMSG_DATA(cmsg) \
((unsigned char *)(((struct cmsghdr *)(cmsg)) + 1))
Taking the address of an rvalue is a hard error in C, so the
current code fails to compile with musl libc.
Remove the erroneous & operator. The resulting code is correct
regardless of the CMSG_DATA implementation -- it simply assigns the
data pointer, which is what the subsequent code (fd_map[--num_fds])
expects.
No functional change with glibc; fixes the build with musl.
Signed-off-by: Marcel W. Wysocki <[email protected]>
---
arch/um/kernel/skas/stub.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/um/kernel/skas/stub.c b/arch/um/kernel/skas/stub.c
--- a/arch/um/kernel/skas/stub.c
+++ b/arch/um/kernel/skas/stub.c
@@ -146,7 +146,7 @@
/* Receive the FDs */
num_fds = 0;
fd_msg = msghdr.msg_control;
- fd_map = (void *)&CMSG_DATA(fd_msg);
+ fd_map = (void *)CMSG_DATA(fd_msg);
if (res == iov.iov_len && msghdr.msg_controllen > sizeof(struct
cmsghdr))
num_fds = (fd_msg->cmsg_len - CMSG_LEN(0)) /
sizeof(int);
