[PATCH 1/2] um: fix address-of CMSG_DATA() rvalue in stub

[PATCH 1/2] um: fix address-of CMSG_DATA() rvalue in stub

[Marcel W. Wysocki]

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>
---
 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);
 

[PATCH 2/2] um: avoid struct sigcontext redefinition with musl

[Marcel W. Wysocki]

mcontext.c includes both <sys/ucontext.h> and <asm/sigcontext.h>.
With musl libc, this causes a struct sigcontext redefinition error:

  <sys/ucontext.h> pulls in musl's <bits/signal.h>, which defines
  struct sigcontext directly.  The kernel's <asm/sigcontext.h> then
  provides a second, conflicting definition of the same struct.

With glibc this does not conflict because glibc's signal headers
source their struct sigcontext from the kernel's own UAPI headers,
so the include guard in <asm/sigcontext.h> makes the second
inclusion a no-op.

mcontext.c does not actually use struct sigcontext by name -- it
only needs the FP-state types (_fpstate, _xstate, etc.) that are
defined in <asm/sigcontext.h> independently of the sigcontext
struct.

Temporarily rename sigcontext to __kernel_sigcontext during the
inclusion of <asm/sigcontext.h> so that the kernel's definition
does not collide with musl's.  The #undef restores normal name
resolution immediately afterward.

No functional change with glibc; fixes the build with musl.

Signed-off-by: Marcel W. Wysocki <maci.stgn@gmail.com>
---
 arch/x86/um/os-Linux/mcontext.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/um/os-Linux/mcontext.c b/arch/x86/um/os-Linux/mcontext.c
--- a/arch/x86/um/os-Linux/mcontext.c
+++ b/arch/x86/um/os-Linux/mcontext.c
@@ -4,7 +4,13 @@
 #include <linux/string.h>
 #include <sys/ucontext.h>
 #include <asm/ptrace.h>
+/*
+ * musl defines struct sigcontext in <bits/signal.h>.  Rename the kernel's
+ * copy to avoid redefinition while keeping the FP-state types available.
+ */
+#define sigcontext __kernel_sigcontext
 #include <asm/sigcontext.h>
+#undef sigcontext
 #include <sysdep/ptrace.h>
 #include <sysdep/mcontext.h>
 #include <arch.h>