mcount-dynamic: Use MAP_FIXED_NOREPLACE for trampoline

By trying to allocate the extra page for a trampoline with MAP_FIXED,
Uftrace could end up overlapping already mapped pages, which are
discarded.  Typically, if data was there with rw- protection, now the
data is undefined and has rwx protection.

Since Linux 4.17, the flag MAP_FIXED_NOREPLACE can be used to ensure
that the fixed location is mapped atomically, preventing any clobbering.

For earlier kernel, the call to mmap(2) will fall back to a random
address, by not honoring the MAP_FIXED request at all.  So callers
should ensure that the returned address matches the requested one.

Signed-off-by: Olivier Dion <odion@efficios.com>

arch/aarch64/mcount-dynamic.c

@@ -63,13 +63,21 @@ int mcount_setup_trampoline(struct mcount_dynamic_info *mdi)
 	mdi->trampoline -= sizeof(trampoline);
 
 	if (unlikely(mdi->trampoline < mdi->text_addr + mdi->text_size)) {
+		void *trampoline_check;
+
 		mdi->trampoline += sizeof(trampoline);
 		mdi->text_size += PAGE_SIZE;
 
 		pr_dbg("adding a page for fentry trampoline at %#lx\n", mdi->trampoline);
 
-		mmap((void *)mdi->trampoline, PAGE_SIZE, PROT_READ | PROT_WRITE | PROT_EXEC,
-		     MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+		trampoline_check = mmap((void *)mdi->trampoline, PAGE_SIZE,
+					PROT_READ | PROT_WRITE | PROT_EXEC,
+					MAP_FIXED_NOREPLACE | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+
+		if (trampoline_check != (void *)mdi->trampoline) {
+			pr_err("could not map trampoline at desired location %#lx, got %#lx: %m\n",
+			       mdi->trampoline, (uintptr_t)trampoline_check);
+		}
 	}
 
 	page_offset = mdi->text_addr & (PAGE_SIZE - 1);

arch/i386/mcount-dynamic.c

@@ -30,10 +30,12 @@ int mcount_setup_trampoline(struct mcount_dynamic_info *mdi)
 		pr_dbg2("adding a page for fentry trampoline at %#lx\n", mdi->trampoline);
 
 		trampoline_check = mmap((void *)mdi->trampoline, PAGE_SIZE, PROT_READ | PROT_WRITE,
-					MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+					MAP_FIXED_NOREPLACE | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
 
-		if (trampoline_check == MAP_FAILED)
-			pr_err("failed to mmap trampoline for setup");
+		if (trampoline_check != (void *)mdi->trampoline) {
+			pr_err("could not map trampoline at desired location %#lx, got %#lx: %m\n",
+			       mdi->trampoline, (uintptr_t)trampoline_check);
+		}
 	}
 
 	if (mprotect((void *)mdi->text_addr, mdi->text_size, PROT_READ | PROT_WRITE)) {

arch/x86_64/mcount-dynamic.c

@@ -43,10 +43,12 @@ int mcount_setup_trampoline(struct mcount_dynamic_info *mdi)
 
 		trampoline_check = mmap((void *)mdi->trampoline, PAGE_SIZE,
 					PROT_READ | PROT_WRITE | PROT_EXEC,
-					MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+					MAP_FIXED_NOREPLACE | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
 
-		if (trampoline_check == MAP_FAILED)
-			pr_err("failed to mmap trampoline for setup");
+		if (trampoline_check != (void *)mdi->trampoline) {
+			pr_err("could not map trampoline at desired location %#lx, got %#lx: %m\n",
+			       mdi->trampoline, (uintptr_t)trampoline_check);
+		}
 	}
 
 	if (mprotect(PAGE_ADDR(mdi->text_addr), PAGE_LEN(mdi->text_addr, mdi->text_size),