aarch64: Re-implement setcontext without rt_sigreturn syscall

The current implementation of setcontext uses rt_sigreturn to restore
the contents of registers. This contrasts with the way most other
architectures implement setcontext:

  powerpc64, mips, tile:

  Call rt_sigreturn if context was created by a call to a signal handler,
  otherwise restore in user code.

  powerpc32:

  Call swapcontext system call and don't call sigreturn or rt_sigreturn.

  x86_64, sparc, hppa, sh, ia64, m68k, s390, arm:

  Only support restoring "synchronous" contexts, that is contexts
  created by getcontext, and restoring in user code and don't call
  sigreturn or rt_sigreturn.

  alpha:

  Call sigreturn (but not rt_sigreturn) in all cases to do the restore.

The text of the setcontext manpage suggests that the requirement to be
able to restore a signal handler created context has been dropped from
SUSv2:

  If  the context was obtained by a call to a signal handler, then old
  standard text says that "program execution continues with the program
  instruction following the instruction interrupted by the signal".
  However, this sentence was removed in SUSv2, and the present verdict
  is "the result is unspecified".

Implementing setcontext by calling rt_sigreturn unconditionally causes
problems when used with sigaltstack as in BZ #16629. On this basis it
seems that aarch64 is broken and that new ports should only support
restoring contexts created with getcontext and do not need to call
rt_sigreturn at all.

This patch re-implements the aarch64 setcontext function to restore
the context in user code in a similar manner to x86_64 and other ports.

ChangeLog:

2014-04-17  Will Newton  <will.newton@linaro.org>

	[BZ #16629]
	* sysdeps/unix/sysv/linux/aarch64/setcontext.S (__setcontext):
	Re-implement to restore registers in user code and avoid
	rt_sigreturn system call.

2 files changed, 101 insertions, 55 deletions

diff --git a/libc/ports/ChangeLog.aarch64.linaro b/libc/ports/ChangeLog.aarch64.linaro
new file mode 100644
index 000000000..be992403a
--- /dev/null
+++ b/libc/ports/ChangeLog.aarch64.linaro
@@ -0,0 +1,6 @@
+2014-04-17  Will Newton  <will.newton@linaro.org>
+
+	[BZ #16629]
+	* sysdeps/unix/sysv/linux/aarch64/setcontext.S (__setcontext):
+	Re-implement to restore registers in user code and avoid
+	rt_sigreturn system call.
diff --git a/libc/ports/sysdeps/unix/sysv/linux/aarch64/setcontext.S b/libc/ports/sysdeps/unix/sysv/linux/aarch64/setcontext.S
index d220c41f6..f45a655a0 100644
--- a/libc/ports/sysdeps/unix/sysv/linux/aarch64/setcontext.S
+++ b/libc/ports/sysdeps/unix/sysv/linux/aarch64/setcontext.S
@@ -22,68 +22,108 @@
 #include "ucontext_i.h"
 #include "ucontext-internal.h"
 
-/* int setcontext (const ucontext_t *ucp) */
+/*  int __setcontext (const ucontext_t *ucp)
 
-	.text
-
-ENTRY(__setcontext)
-
-	/* Create a signal frame on the stack:
-
-		fp
-		lr
-		...
-	   sp-> rt_sigframe
-	 */
-
-	stp     x29, x30, [sp, -16]!
-	cfi_adjust_cfa_offset (16)
-	cfi_rel_offset (x29, 0)
-	cfi_rel_offset (x30, 8)
-
-        mov     x29, sp
-	cfi_def_cfa_register (x29)
-
-	/* Allocate space for the sigcontext.  */
-	mov	w3, #((RT_SIGFRAME_SIZE + SP_ALIGN_SIZE) & SP_ALIGN_MASK)
-	sub	sp, sp,	x3
+  Restores the machine context in UCP and thereby resumes execution
+  in that context.
 
-	/* Compute the base address of the ucontext structure.  */
-	add	x1, sp, #RT_SIGFRAME_UCONTEXT
+  This implementation is intended to be used for *synchronous* context
+  switches only.  Therefore, it does not have to restore anything
+  other than the PRESERVED state.  */
 
-	/* Only ucontext is required in the frame, *copy* it in.  */
-
-#if UCONTEXT_SIZE % 16
-#error The implementation of setcontext.S assumes sizeof(ucontext_t) % 16 == 0
-#endif
-
-	mov	x2, #UCONTEXT_SIZE / 16
-0:
-	ldp	x3, x4, [x0], #16
-	stp	x3, x4, [x1], #16
-	sub	x2, x2, 1
-	cbnz	x2, 0b
+	.text
 
-	/* rt_sigreturn () -- no arguments, sp points to struct rt_sigframe.  */
-	mov	x8, SYS_ify (rt_sigreturn)
+ENTRY (__setcontext)
+	/* Save a copy of UCP.  */
+	mov	x9, x0
+
+	/* Set the signal mask with
+	   rt_sigprocmask (SIG_SETMASK, mask, NULL, _NSIG/8).  */
+	mov	x0, #SIG_SETMASK
+	add	x1, x9, #UCONTEXT_SIGMASK
+	mov	x2, #0
+	mov	x3, #_NSIG8
+	mov	x8, SYS_ify (rt_sigprocmask)
 	svc	0
-
-	/* Ooops we failed.  Recover the stack */
-
-	mov	sp, x29
-	cfi_def_cfa_register (sp)
-
-        ldp     x29, x30, [sp], 16
-	cfi_adjust_cfa_offset (16)
-	cfi_restore (x29)
-	cfi_restore (x30)
-	b	C_SYMBOL_NAME(__syscall_error)
-
+	cbz	x0, 1f
+	b	C_SYMBOL_NAME (__syscall_error)
+1:
+	/* Restore the general purpose registers.  */
+	mov	x0, x9
+	cfi_def_cfa (x0, 0)
+	cfi_offset (x18, oX0 + 18 * SZREG)
+	cfi_offset (x19, oX0 + 19 * SZREG)
+	cfi_offset (x20, oX0 + 20 * SZREG)
+	cfi_offset (x21, oX0 + 21 * SZREG)
+	cfi_offset (x22, oX0 + 22 * SZREG)
+	cfi_offset (x23, oX0 + 23 * SZREG)
+	cfi_offset (x24, oX0 + 24 * SZREG)
+	cfi_offset (x25, oX0 + 25 * SZREG)
+	cfi_offset (x26, oX0 + 26 * SZREG)
+	cfi_offset (x27, oX0 + 27 * SZREG)
+	cfi_offset (x28, oX0 + 28 * SZREG)
+	cfi_offset (x29, oX0 + 29 * SZREG)
+	cfi_offset (x30, oX0 + 30 * SZREG)
+
+	cfi_offset ( d8, oV0 + 8 * SZVREG)
+	cfi_offset ( d9, oV0 + 9 * SZVREG)
+	cfi_offset (d10, oV0 + 10 * SZVREG)
+	cfi_offset (d11, oV0 + 11 * SZVREG)
+	cfi_offset (d12, oV0 + 12 * SZVREG)
+	cfi_offset (d13, oV0 + 13 * SZVREG)
+	cfi_offset (d14, oV0 + 14 * SZVREG)
+	cfi_offset (d15, oV0 + 15 * SZVREG)
+	ldp	x18, x19, [x0, oX0 + 18 * SZREG]
+	ldp	x20, x21, [x0, oX0 + 20 * SZREG]
+	ldp	x22, x23, [x0, oX0 + 22 * SZREG]
+	ldp	x24, x25, [x0, oX0 + 24 * SZREG]
+	ldp	x26, x27, [x0, oX0 + 26 * SZREG]
+	ldp	x28, x29, [x0, oX0 + 28 * SZREG]
+	ldr     x30,      [x0, oX0 + 30 * SZREG]
+	ldr     x2, [x0, oSP]
+	mov	sp, x2
+
+	/* Check for FP SIMD context.  We don't support restoring
+	   contexts created by the kernel, so this context must have
+	   been created by getcontext.  Hence we can rely on the
+	   first extension block being the FP SIMD context.  */
+	add     x2, x0, #oEXTENSION
+
+	mov	w3, #(FPSIMD_MAGIC & 0xffff)
+	movk	w3, #(FPSIMD_MAGIC >> 16), lsl #16
+	ldr	w1, [x2, #oHEAD + oMAGIC]
+	cmp	w1, w3
+	b.ne	2f
+
+	/* Restore the FP SIMD context.  */
+	add	x3, x2, #oV0 + 8 * SZVREG
+	ldp	 d8,  d9, [x3], #2 * SZVREG
+	ldp	d10, d11, [x3], #2 * SZVREG
+	ldp	d12, d13, [x3], #2 * SZVREG
+	ldp	d14, d15, [x3], #2 * SZVREG
+
+	add	x3, x2, oFPSR
+
+	ldr	w4, [x3]
+	msr	fpsr, x4
+
+	ldr	w4, [x3, oFPCR - oFPSR]
+	msr	fpcr, x4
+
+2:
+	ldr     x16, [x0, oPC]
+	/* Restore arg registers.  */
+	ldp	x2, x3, [x0, oX0 + 2 * SZREG]
+	ldp	x4, x5, [x0, oX0 + 4 * SZREG]
+	ldp	x6, x7, [x0, oX0 + 6 * SZREG]
+	ldp	x0, x1, [x0, oX0 + 0 * SZREG]
+	/* Jump to the new pc value.  */
+	br	x16
 PSEUDO_END (__setcontext)
 weak_alias (__setcontext, setcontext)
 
-ENTRY(__startcontext)
+ENTRY (__startcontext)
 	mov	x0, x19
 	cbnz	x0, __setcontext
-1:	b       HIDDEN_JUMPTARGET(_exit)
-END(__startcontext)
+1:	b       HIDDEN_JUMPTARGET (_exit)
+END (__startcontext)