x86/asm/entry/64: Fix comments

- Misleading and slightly incorrect comments in "struct pt_regs" are
   fixed (four instances).

 - Fix incorrect comment atop EMPTY_FRAME macro.

 - Explain in more detail what we do with stack layout during hw interrupt.

 - Correct comments about "partial stack frame" which are no longer
   true.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1423778052-21038-3-git-send-email-dvlasenk@redhat.com
Link: http://lkml.kernel.org/r/e1f4429c491fe6ceeddb879dea2786e0f8920f9c.1424989793.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/ptrace.h

@@ -31,13 +31,17 @@ struct pt_regs {
 #else /* __i386__ */
 
 struct pt_regs {
+/*
+ * C ABI says these regs are callee-preserved. They aren't saved on kernel entry
+ * unless syscall needs a complete, fully filled "struct pt_regs".
+ */
 	unsigned long r15;
 	unsigned long r14;
 	unsigned long r13;
 	unsigned long r12;
 	unsigned long bp;
 	unsigned long bx;
-/* arguments: non interrupts/non tracing syscalls only save up to here*/
+/* These regs are callee-clobbered. Always saved on kernel entry. */
 	unsigned long r11;
 	unsigned long r10;
 	unsigned long r9;
@@ -47,9 +51,12 @@ struct pt_regs {
 	unsigned long dx;
 	unsigned long si;
 	unsigned long di;
+/*
+ * On syscall entry, this is syscall#. On CPU exception, this is error code.
+ * On hw interrupt, it's IRQ number:
+ */
 	unsigned long orig_ax;
-/* end of arguments */
+/* Return frame for iretq */
-/* cpu exception frame or undefined */
 	unsigned long ip;
 	unsigned long cs;
 	unsigned long flags;

arch/x86/include/uapi/asm/ptrace-abi.h

@@ -25,13 +25,17 @@
 #else /* __i386__ */
 
 #if defined(__ASSEMBLY__) || defined(__FRAME_OFFSETS)
+/*
+ * C ABI says these regs are callee-preserved. They aren't saved on kernel entry
+ * unless syscall needs a complete, fully filled "struct pt_regs".
+ */
 #define R15 0
 #define R14 8
 #define R13 16
 #define R12 24
 #define RBP 32
 #define RBX 40
-/* arguments: interrupts/non tracing syscalls only save up to here*/
+/* These regs are callee-clobbered. Always saved on kernel entry. */
 #define R11 48
 #define R10 56
 #define R9 64
@@ -41,9 +45,12 @@
 #define RDX 96
 #define RSI 104
 #define RDI 112
-#define ORIG_RAX 120       /* = ERROR */
+/*
-/* end of arguments */
+ * On syscall entry, this is syscall#. On CPU exception, this is error code.
-/* cpu exception frame or undefined in case of fast syscall. */
+ * On hw interrupt, it's IRQ number:
+ */
+#define ORIG_RAX 120
+/* Return frame for iretq */
 #define RIP 128
 #define CS 136
 #define EFLAGS 144

arch/x86/include/uapi/asm/ptrace.h

@@ -41,13 +41,17 @@ struct pt_regs {
 #ifndef __KERNEL__
 
 struct pt_regs {
+/*
+ * C ABI says these regs are callee-preserved. They aren't saved on kernel entry
+ * unless syscall needs a complete, fully filled "struct pt_regs".
+ */
 	unsigned long r15;
 	unsigned long r14;
 	unsigned long r13;
 	unsigned long r12;
 	unsigned long rbp;
 	unsigned long rbx;
-/* arguments: non interrupts/non tracing syscalls only save up to here*/
+/* These regs are callee-clobbered. Always saved on kernel entry. */
 	unsigned long r11;
 	unsigned long r10;
 	unsigned long r9;
@@ -57,9 +61,12 @@ struct pt_regs {
 	unsigned long rdx;
 	unsigned long rsi;
 	unsigned long rdi;
+/*
+ * On syscall entry, this is syscall#. On CPU exception, this is error code.
+ * On hw interrupt, it's IRQ number:
+ */
 	unsigned long orig_rax;
-/* end of arguments */
+/* Return frame for iretq */
-/* cpu exception frame or undefined */
 	unsigned long rip;
 	unsigned long cs;
 	unsigned long eflags;

arch/x86/kernel/entry_64.S

@@ -14,9 +14,6 @@
  * NOTE: This code handles signal-recognition, which happens every time
  * after an interrupt and after each system call.
  *
- * Normal syscalls and interrupts don't save a full stack frame, this is
- * only done for syscall tracing, signals or fork/exec et.al.
- *
  * A note on terminology:
  * - top of stack: Architecture defined interrupt frame from SS to RIP
  * at the top of the kernel process stack.
@@ -151,7 +148,7 @@ ENDPROC(native_usergs_sysret64)
 	.endm
 
 /*
- * initial frame state for interrupts (and exceptions without error code)
+ * empty frame
  */
 	.macro EMPTY_FRAME start=1 offset=0
 	.if \start
@@ -379,7 +376,7 @@ tracesys_phase2:
 	call syscall_trace_enter_phase2
 
 	/*
-	 * Reload arg registers from stack in case ptrace changed them.
+	 * Reload registers from stack in case ptrace changed them.
 	 * We don't reload %rax because syscall_trace_entry_phase2() returned
 	 * the value it wants us to use in the table lookup.
 	 */
@@ -629,6 +626,13 @@ END(interrupt)
 /* 0(%rsp): ~(interrupt number) */
 	.macro interrupt func
 	cld
+	/*
+	 * Since nothing in interrupt handling code touches r12...r15 members
+	 * of "struct pt_regs", and since interrupts can nest, we can save
+	 * four stack slots and simultaneously provide
+	 * an unwind-friendly stack layout by saving "truncated" pt_regs
+	 * exactly up to rbp slot, without these members.
+	 */
 	ALLOC_PT_GPREGS_ON_STACK -RBP
 	SAVE_C_REGS -RBP
 	/* this goes to 0(%rsp) for unwinder, not for saving the value: */
@@ -641,6 +645,7 @@ END(interrupt)
 	SWAPGS
 1:
 	/*
+	 * Save previous stack pointer, optionally switch to interrupt stack.
 	 * irq_count is used to check if a CPU is already on an interrupt stack
 	 * or not. While this is essentially redundant with preempt_count it is
 	 * a little cheaper to use a separate counter in the PDA (short of
@@ -681,6 +686,7 @@ ret_from_intr:
 	/* Restore saved previous stack */
 	popq %rsi
 	CFI_DEF_CFA rsi,SS+8-RBP	/* reg/off reset after def_cfa_expr */
+	/* return code expects complete pt_regs - adjust rsp accordingly: */
 	leaq ARGOFFSET-RBP(%rsi), %rsp
 	CFI_DEF_CFA_REGISTER	rsp
 	CFI_ADJUST_CFA_OFFSET	RBP-ARGOFFSET
@@ -692,7 +698,7 @@ exit_intr:
 
 	/* Interrupt came from user space */
 	/*
-	 * Has a correct top of stack, but a partial stack frame
+	 * Has a correct top of stack.
 	 * %rcx: thread info. Interrupts off.
 	 */
 retint_with_reschedule: