dtrace: reintroduce one old typedef for userspace ABI compatibility

The conversion of all struct typedefs to typedefs triggered a problem:
DTrace userspace declares the type of the built-in variable curcpu to
be "vmlinux`cpuinfo_t *".  This declaration is hardwired into the source
code: changing it would produce a DTrace that didn't work with older
(non-de-typedeffed) kernels.

These kernels *do* contain a reference to 'struct cpuinfo', but
unfortunately there is no pointer to it declared in the DWARF, only
a pointer to its typedef, so declaring curcpu to be of type
"vmlinux`struct cpuinfo *" doesn't work.

So reintroduce this typedef, and force a pointer to it to be used in one
place in vmlinux (in dtrace_cpu_init()) to ensure that it gets emitted
into the DWARF.

The typedef is not used anywhere else in the kernel: everything else
uses struct cpuinfo, as it should.

Signed-off-by: Nick Alcock <[email protected]>

Author: nickalcock

include/linux/dtrace_cpu.h

@@ -44,6 +44,9 @@ struct cpuinfo {
 
 DECLARE_PER_CPU_SHARED_ALIGNED(struct cpuinfo, dtrace_cpu_info);
 
+/* ABI requirement: type names compiled into DTrace userspace.  */
+typedef struct cpuinfo cpuinfo_t;
+
 extern void dtrace_cpu_init(void);
 
 #endif /* CONFIG_DTRACE */

kernel/dtrace/dtrace_cpu.c

@@ -30,6 +30,12 @@ void dtrace_cpu_init(void)
 {
 	int	cpu;
 
+	/*
+	 * Force this type into the CTF for the sake of userspace's
+	 * ABI requirements.
+	 */
+	cpuinfo_t *dummy __attribute__((__unused__)) = NULL;
+
 	for_each_present_cpu(cpu) {
 		cpuinfo_arch_t		*ci = &cpu_data(cpu);
 		struct cpuinfo		*cpui = per_cpu_info(cpu);