src: "Fix" the system call intrinsic for FreeBSD

FreeBSD's systemcall handler clears out R8, R9, and R10 prior to
`sysretq`, and additionally returns positive errno (with CF) set on
error.  This modifies the syscall intrinsic such that LLVM knows
about the additional clobbered registers.

Note that propagating CF back to the caller of the syscall intrinsic
is left for a future PR.  As far as I can tell, Darwin does not use
the syscall intrinsic at all, and FreeBSD only uses it for SYS_GETTID,
so this should be "ok" for now.

See: sys/amd64/amd64/exception.S in the FreeBSD src for more details.

src/llvm_backend_proc.cpp

@@ -1985,11 +1985,27 @@ lbValue lb_build_builtin_proc(lbProcedure *p, Ast *expr, TypeAndValue const &tv,
 			case TargetArch_amd64:
 				{
 					GB_ASSERT(arg_count <= 7);
-					
+
+					// FreeBSD additionally clobbers r8, r9, r10, but they
+					// can also be used to pass in arguments, so this needs
+					// to be handled in two parts.
+					bool clobber_arg_regs[7] = {
+						false, false, false, false, false, false, false
+					};
+					if (build_context.metrics.os == TargetOs_freebsd) {
+						clobber_arg_regs[4] = true; // r10
+						clobber_arg_regs[5] = true; // r8
+						clobber_arg_regs[6] = true; // r9
+					}
+
 					char asm_string[] = "syscall";
 					gbString constraints = gb_string_make(heap_allocator(), "={rax}");
 					for (unsigned i = 0; i < arg_count; i++) {
-						constraints = gb_string_appendc(constraints, ",{");
+						if (!clobber_arg_regs[i]) {
+							constraints = gb_string_appendc(constraints, ",{");
+						} else {
+							constraints = gb_string_appendc(constraints, ",+{");
+						}
 						static char const *regs[] = {
 							"rax",
 							"rdi",
@@ -2013,10 +2029,35 @@ lbValue lb_build_builtin_proc(lbProcedure *p, Ast *expr, TypeAndValue const &tv,
 					// Some but not all system calls will additionally
 					// clobber memory.
 					//
-					// TODO: FreeBSD is different and will also clobber
-					// R8, R9, and R10.  Additionally CF is used to
-					// indicate an error instead of -errno.
+					// As a fix for CVE-2019-5595, FreeBSD started
+					// clobbering R8, R9, and R10, instead of restoring
+					// them.  Additionally unlike Linux, instead of
+					// returning negative errno, positive errno is
+					// returned and CF is set.
+					//
+					// TODO:
+					//  * Figure out what Darwin does.
+					//  * Add some extra handling to propagate CF back
+					//    up to the caller on FreeBSD systems so that
+					//    the caller knows that the return value is
+					//    positive errno.
 					constraints = gb_string_appendc(constraints, ",~{rcx},~{r11},~{memory}");
+					if (build_context.metrics.os == TargetOs_freebsd) {
+						// Second half of dealing with FreeBSD's system
+						// call semantics.  Explicitly clobber the registers
+						// that were not used to pass in arguments, and
+						// then clobber RFLAGS.
+						if (arg_count < 5) {
+							constraints = gb_string_appendc(constraints, ",~{r10}");
+						}
+						if (arg_count < 6) {
+							constraints = gb_string_appendc(constraints, ",~{r8}");
+						}
+						if (arg_count < 7) {
+							constraints = gb_string_appendc(constraints, ",~{r9}");
+						}
+						constraints = gb_string_appendc(constraints, ",~{cc}");
+					}
 
 					inline_asm = llvm_get_inline_asm(func_type, make_string_c(asm_string), make_string_c(constraints));
 				}