github-mirror/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-05-30 18:13:41 +02:00
Code Issues Actions Packages Projects Releases Wiki Activity Graph

um: fix address-of CMSG_DATA() rvalue in stub

Browse Source 
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 <maci.stgn@gmail.com>
Link: https://patch.msgid.link/20260215142803.1455757-1-maci.stgn@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
This commit is contained in:
Marcel W. Wysocki 2026-02-15 22:28:02 +08:00 committed by Johannes Berg
parent f338e77383
commit 4076f73298
1 changed files with 1 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
arch/um/kernel/skas/stub.c
View File

@ -146,7 +146,7 @@ stub_signal_interrupt(int sig, siginfo_t *info, void *p)
/* 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);