properly access mcontext_t program counter in cancellation handler

using the actual mcontext_t definition rather than an overlaid pointer
array both improves correctness/readability and eliminates some ugly
hacks for archs with 64-bit registers bit 32-bit program counter.

also fix UB due to comparison of pointers not in a common array
object.

arch/aarch64/pthread_arch.h

@@ -8,4 +8,4 @@ static inline struct pthread *__pthread_self()
 #define TLS_ABOVE_TP
 #define TP_ADJ(p) ((char *)(p) + sizeof(struct pthread) - 16)
 
-#define CANCEL_REG_IP 33
+#define MC_PC pc

arch/arm/pthread_arch.h

@@ -27,4 +27,4 @@ static inline pthread_t __pthread_self()
 #define TLS_ABOVE_TP
 #define TP_ADJ(p) ((char *)(p) + sizeof(struct pthread) - 8)
 
-#define CANCEL_REG_IP 18
+#define MC_PC arm_pc

arch/i386/pthread_arch.h

@@ -7,4 +7,4 @@ static inline struct pthread *__pthread_self()
 
 #define TP_ADJ(p) (p)
 
-#define CANCEL_REG_IP 14
+#define MC_PC gregs[REG_EIP]

arch/microblaze/pthread_arch.h

@@ -7,4 +7,4 @@ static inline struct pthread *__pthread_self()
 
 #define TP_ADJ(p) (p)
 
-#define CANCEL_REG_IP 32
+#define MC_PC regs.pc

arch/mips/pthread_arch.h

@@ -16,4 +16,4 @@ static inline struct pthread *__pthread_self()
 
 #define DTP_OFFSET 0x8000
 
-#define CANCEL_REG_IP (3-(union {int __i; char __b;}){1}.__b)
+#define MC_PC pc

arch/or1k/pthread_arch.h

@@ -14,5 +14,4 @@ static inline struct pthread *__pthread_self()
 #define TLS_ABOVE_TP
 #define TP_ADJ(p) ((char *)(p) + sizeof(struct pthread))
 
-/* word-offset to 'pc' in mcontext_t */
-#define CANCEL_REG_IP 32
+#define MC_PC regs.pc

arch/powerpc/pthread_arch.h

@@ -15,9 +15,8 @@ static inline struct pthread *__pthread_self()
 
 #define DTP_OFFSET 0x8000
 
-// offset of the PC register in mcontext_t, divided by the system wordsize
 // the kernel calls the ip "nip", it's the first saved value after the 32
 // GPRs.
-#define CANCEL_REG_IP 32
+#define MC_PC gregs[32]
 
 #define CANARY canary_at_end

arch/sh/pthread_arch.h

@@ -8,4 +8,4 @@ static inline struct pthread *__pthread_self()
 #define TLS_ABOVE_TP
 #define TP_ADJ(p) ((char *)(p) + sizeof(struct pthread) - 8)
 
-#define CANCEL_REG_IP 17
+#define MC_PC sc_pc

arch/x32/pthread_arch.h

@@ -7,6 +7,6 @@ static inline struct pthread *__pthread_self()
 
 #define TP_ADJ(p) (p)
 
-#define CANCEL_REG_IP 32
+#define MC_PC gregs[REG_RIP]
 
 #define CANARY canary2

arch/x86_64/pthread_arch.h

@@ -7,4 +7,4 @@ static inline struct pthread *__pthread_self()
 
 #define TP_ADJ(p) (p)
 
-#define CANCEL_REG_IP 16
+#define MC_PC gregs[REG_RIP]

src/thread/pthread_cancel.c

@@ -1,3 +1,4 @@
+#define _GNU_SOURCE
 #include <string.h>
 #include "pthread_impl.h"
 #include "syscall.h"
@@ -61,15 +62,15 @@ static void cancel_handler(int sig, siginfo_t *si, void *ctx)
 {
 	pthread_t self = __pthread_self();
 	ucontext_t *uc = ctx;
-	const char *ip = ((char **)&uc->uc_mcontext)[CANCEL_REG_IP];
+	uintptr_t pc = uc->uc_mcontext.MC_PC;
 
 	a_barrier();
 	if (!self->cancel || self->canceldisable == PTHREAD_CANCEL_DISABLE) return;
 
 	_sigaddset(&uc->uc_sigmask, SIGCANCEL);
 
-	if (self->cancelasync || ip >= __cp_begin && ip < __cp_end) {
-		((char **)&uc->uc_mcontext)[CANCEL_REG_IP] = (char *)__cp_cancel;
+	if (self->cancelasync || pc >= (uintptr_t)__cp_begin && pc < (uintptr_t)__cp_end) {
+		uc->uc_mcontext.MC_PC = (uintptr_t)__cp_cancel;
 		return;
 	}