John Bowler writes in Bug 385:

This patch changes all cases where the ARM assembler mov pc,rx
instructions are used to ensure that the thumb/arm interwork change of
process more works - in essence mov pc,rx needs to become bx rc.

The ldr pc or ldm rx, {pc} instructions are not changed - this is
fine on ARM >=v5 but will fail to restore thumb mode on ARM v4T,
i.e. this code will not provide support for thumb on ARM v4T.

One mov pc is left in resolve.S, this is fixed in a different patch -
thumb-resolve.patch

The changes are protected by __THUMB_INTERWORK__ - the original
mov instruction will work on newer architectures and is required on
arch v4 (not v4t) and earlier - those which did not support thumb -
so this is safe.  See gcc lib1asmfuncs for a more exact test.

2 files changed, 13 insertions, 1 deletions

diff --git a/libc/sysdeps/linux/arm/clone.S b/libc/sysdeps/linux/arm/clone.S
index 6672c7d6e..98b1296c0 100644
--- a/libc/sysdeps/linux/arm/clone.S
+++ b/libc/sysdeps/linux/arm/clone.S
@@ -52,7 +52,11 @@ clone:
 	DO_CALL (clone)
 	movs	a1, a1
 	blt	__error
-	movne    pc, lr
+#if defined(__THUMB_INTERWORK__)
+	bxne	lr
+#else
+	movne	pc, lr
+#endif
 
 	@ pick the function arg and call address off the stack and execute
 	ldr	r0, [sp, #4]
diff --git a/libc/sysdeps/linux/arm/vfork.S b/libc/sysdeps/linux/arm/vfork.S
index e13636940..486a7e8e4 100644
--- a/libc/sysdeps/linux/arm/vfork.S
+++ b/libc/sysdeps/linux/arm/vfork.S
@@ -26,7 +26,11 @@ __vfork:
 #ifdef __NR_vfork
 	DO_CALL (vfork)
 	cmn	r0, #4096
+#if defined(__THUMB_INTERWORK__)
+	bxcc	lr
+#else
 	movcc	pc, lr
+#endif
 
 	/* Check if vfork even exists.  */
 	ldr     r1, =-ENOSYS
@@ -39,7 +43,11 @@ __vfork:
 	cmn     r0, #4096
 
 	/* Syscall worked.  Return to child/parent */
+#if defined(__THUMB_INTERWORK__)
+	bxcc	lr
+#else
 	movcc   pc, lr
+#endif
 
 __error:
 	b	__syscall_error