aarch64: define struct user_pac_mask if needed

On Aarch64, Linux is using Pointer Authentication Code (PAC) for pointer
authentication.[1] The struct "user_pac_mask" has been part of the Linux
kernel since version 5.0 as part of this feature. However, older kernels
do not define it.

Therefore, we want to check if the definition is present in the kernel
headers and provide one if it isn't. This ensures two things:

* elfutils will continue to compile against kernel headers from 4.x
* binaries built against older kernel headers will still be fully
  functional if used on a newer system

For reference, the build error that is being avoided looks like this:

[...]
  CC       aarch64_initreg.o
aarch64_initreg.c: In function 'aarch64_set_initial_registers_tid':
aarch64_initreg.c:61:24: error: storage size of 'pac_mask' isn't known
   struct user_pac_mask pac_mask;
                        ^~~~~~~~
aarch64_initreg.c:61:24: warning: unused variable 'pac_mask' [-Wunused-variable]
make[4]: *** [Makefile:831: aarch64_initreg.o] Error 1
make[3]: *** [Makefile:547: all-recursive] Error 1
make[2]: *** [Makefile:463: all] Error 2

[1] https://docs.kernel.org/arch/arm64/pointer-authentication.html

https://sourceware.org/bugzilla/show_bug.cgi?id=32684

Fixes: 64e3b451ad2c ("aarch64: extend dwfl_thread_state_registers to handle PAC")
Signed-off-by: Markus Mayer <[email protected]>

2 files changed, 29 insertions, 0 deletions

diff --git a/backends/aarch64_initreg.c b/backends/aarch64_initreg.c
index 5ec45ea6..a6badbb4 100644
--- a/backends/aarch64_initreg.c
+++ b/backends/aarch64_initreg.c
@@ -47,6 +47,27 @@
 #define BACKEND aarch64_
 #include "libebl_CPU.h"
 
+/*
+ * pointer authentication masks (NT_ARM_PAC_MASK)
+ *
+ * Defined by Linux kernel headers since Linux 5.0. Define it here if kernel
+ * headers are older than that, to ensure this file builds regardless.
+ */
+#if defined(__aarch64__) && defined(__linux__)
+
+#ifndef NT_ARM_PAC_MASK
+#define NT_ARM_PAC_MASK 0x406
+#endif
+
+#ifndef HAVE_USER_PACK_MASK
+struct user_pac_mask {
+  __u64 data_mask;
+  __u64 insn_mask;
+};
+#endif
+
+#endif /* __aarch64__ && __linux__ */
+
 bool
 aarch64_set_initial_registers_tid (pid_t tid __attribute__ ((unused)),
 			  ebl_tid_registers_t *setfunc __attribute__ ((unused)),
diff --git a/configure.ac b/configure.ac
index e57d3927..3298f7fc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -777,6 +777,14 @@ if test "$sys_user_has_user_regs" = "yes"; then
             [Define to 1 if <sys/user.h> defines struct user_regs_struct])
 fi
 
+AC_CHECK_TYPE([struct user_pac_mask],
+              [has_user_pac_mask=yes], [has_user_pac_mask=no],
+              [[#include <asm/ptrace.h>]])
+if test "$has_user_pac_mask" = "yes"; then
+  AC_DEFINE(HAVE_USER_PACK_MASK, 1,
+            [Defined if struct user_pac_mask exists.])
+fi
+
 # On a 64-bit host where can can use $CC -m32, we'll run two sets of tests.
 utrace_BIARCH
 CC_BIARCH="$CC $utrace_biarch"