[tip:x86/asm] x86/asm/entry/64: Fix comments

From: tip-bot for Denys Vlasenko
Date: Wed Mar 04 2015 - 17:54:23 EST

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Commit-ID: e90e147cbc0cbc8dcf48000e15190badf75250f4
Gitweb: http://git.kernel.org/tip/e90e147cbc0cbc8dcf48000e15190badf75250f4
Author: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
AuthorDate: Thu, 26 Feb 2015 14:40:28 -0800
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 4 Mar 2015 22:50:49 +0100

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@xxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Will Drewry <wad@xxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1423778052-21038-3-git-send-email-dvlasenk@xxxxxxxxxx
Link: http://lkml.kernel.org/r/e1f4429c491fe6ceeddb879dea2786e0f8920f9c.1424989793.git.luto@xxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
arch/x86/include/asm/ptrace.h | 13 ++++++++++---
arch/x86/include/uapi/asm/ptrace-abi.h | 15 +++++++++++----
arch/x86/include/uapi/asm/ptrace.h | 13 ++++++++++---
arch/x86/kernel/entry_64.S | 18 ++++++++++++------
4 files changed, 43 insertions(+), 16 deletions(-)

diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index 86fc2bb..4077d96 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/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 */
-/* cpu exception frame or undefined */
+/* Return frame for iretq */
unsigned long ip;
unsigned long cs;
unsigned long flags;
diff --git a/arch/x86/include/uapi/asm/ptrace-abi.h b/arch/x86/include/uapi/asm/ptrace-abi.h
index ad115bf..580aee3 100644
--- a/arch/x86/include/uapi/asm/ptrace-abi.h
+++ b/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 */
-/* cpu exception frame or undefined in case of fast syscall. */
+/*
+ * On syscall entry, this is syscall#. On CPU exception, this is error code.
+ * On hw interrupt, it's IRQ number:
+ */
+#define ORIG_RAX 120
+/* Return frame for iretq */
#define RIP 128
#define CS 136
#define EFLAGS 144
diff --git a/arch/x86/include/uapi/asm/ptrace.h b/arch/x86/include/uapi/asm/ptrace.h
index ac4b9aa..bc16115 100644
--- a/arch/x86/include/uapi/asm/ptrace.h
+++ b/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 */
-/* cpu exception frame or undefined */
+/* Return frame for iretq */
unsigned long rip;
unsigned long cs;
unsigned long eflags;
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index e8372e0..695f4d4 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/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:
--
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Next message: tip-bot for Denys Vlasenko: "[tip:x86/asm] x86/asm/entry/64: Shrink code in 'paranoid_exit'"
Previous message: tip-bot for Denys Vlasenko: "[tip:x86/asm] x86/asm/entry/64: Always allocate a complete " struct pt_regs" on the kernel stack"
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