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Subject[PATCH 4/6 v3] ftrace/x86_32: Clean up ftrace_regs_caller
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

When ftrace_regs_caller was created, it was designed to preserve flags as
much as possible as it needed to act just like a breakpoint triggered on the
same location. But the design is over complicated as it treated all
operations as modifying flags. But push, mov and lea do not modify flags.
This means the code can become more simplified by allowing flags to be
stored further down.

Making ftrace_regs_caller simpler will also be useful in implementing fentry
logic.

Link: http://lkml.kernel.org/r/20170316135328.36123c3e@gandalf.local.home
[ simpler logic to save flags ]
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
arch/x86/kernel/ftrace_32.S | 40 +++++++++++++++++++++-------------------
1 file changed, 21 insertions(+), 19 deletions(-)

diff --git a/arch/x86/kernel/ftrace_32.S b/arch/x86/kernel/ftrace_32.S
index f991e723c3e4..7cd27cf56578 100644
--- a/arch/x86/kernel/ftrace_32.S
+++ b/arch/x86/kernel/ftrace_32.S
@@ -56,23 +56,27 @@ WEAK(ftrace_stub)
END(ftrace_caller)

ENTRY(ftrace_regs_caller)
- pushf /* push flags before compare (in cs location) */
-
/*
* i386 does not save SS and ESP when coming from kernel.
* Instead, to get sp, &regs->sp is used (see ptrace.h).
* Unfortunately, that means eflags must be at the same location
* as the current return ip is. We move the return ip into the
- * ip location, and move flags into the return ip location.
+ * regs->ip location, and move flags into the return ip location.
*/
- pushl 4(%esp) /* save return ip into ip slot */
-
+ pushl $__KERNEL_CS
+ pushl 4(%esp) /* Save the return ip */
pushl $0 /* Load 0 into orig_ax */
pushl %gs
pushl %fs
pushl %es
pushl %ds
pushl %eax
+
+ /* Get flags and place them into the return ip slot */
+ pushf
+ popl %eax
+ movl %eax, 8*4(%esp)
+
pushl %ebp
pushl %edi
pushl %esi
@@ -80,11 +84,6 @@ ENTRY(ftrace_regs_caller)
pushl %ecx
pushl %ebx

- movl 13*4(%esp), %eax /* Get the saved flags */
- movl %eax, 14*4(%esp) /* Move saved flags into regs->flags location */
- /* clobbering return ip */
- movl $__KERNEL_CS, 13*4(%esp)
-
movl 12*4(%esp), %eax /* Load ip (1st parameter) */
subl $MCOUNT_INSN_SIZE, %eax /* Adjust ip */
movl 0x4(%ebp), %edx /* Load parent ip (2nd parameter) */
@@ -95,10 +94,14 @@ GLOBAL(ftrace_regs_call)
call ftrace_stub

addl $4, %esp /* Skip pt_regs */
- movl 14*4(%esp), %eax /* Move flags back into cs */
- movl %eax, 13*4(%esp) /* Needed to keep addl from modifying flags */
- movl 12*4(%esp), %eax /* Get return ip from regs->ip */
- movl %eax, 14*4(%esp) /* Put return ip back for ret */
+
+ /* restore flags */
+ push 14*4(%esp)
+ popf
+
+ /* Move return ip back to its original location */
+ movl 12*4(%esp), %eax
+ movl %eax, 14*4(%esp)

popl %ebx
popl %ecx
@@ -111,12 +114,11 @@ GLOBAL(ftrace_regs_call)
popl %es
popl %fs
popl %gs
- addl $8, %esp /* Skip orig_ax and ip */
- popf /* Pop flags at end (no addl to corrupt flags) */
- jmp .Lftrace_ret

- popf
- jmp ftrace_stub
+ /* use lea to not affect flags */
+ lea 3*4(%esp), %esp /* Skip orig_ax, ip and flags */
+
+ jmp .Lftrace_ret
#else /* ! CONFIG_DYNAMIC_FTRACE */

ENTRY(mcount)
--
2.10.2

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