[PATCH] Use %ebp rather than %ebx for thread_info pointer

[PATCH] Use %ebp rather than %ebx for thread_info pointer

[Luca Barbieri]

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This patch changes assembly code that accesses thread_info to use %ebp
rather than %ebx. 

This allows me to take advantage of the fact that %ebp is restored by
user mode in the sysenter register pop removal patch.


diff --exclude-from=/home/ldb/src/exclude -urNdp linux-2.5.54-preldb/arch/i386/kernel/entry.S linux-2.5.54-ldb/arch/i386/kernel/entry.S
--- linux-2.5.54-preldb/arch/i386/kernel/entry.S	2003-01-06 16:01:40.000000000 +0100
+++ linux-2.5.54-ldb/arch/i386/kernel/entry.S	2003-01-06 04:54:58.000000000 +0100
@@ -145,16 +145,16 @@ ENTRY(lcall7)
 				# gates, which has to be cleaned up later..
 	pushl %eax
 	SAVE_ALL
-	movl %esp, %ebx
-	pushl %ebx
+	movl %esp, %ebp
+	pushl %ebp
 	pushl $0x7
 do_lcall:
-	movl EIP(%ebx), %eax	# due to call gates, this is eflags, not eip..
-	movl CS(%ebx), %edx	# this is eip..
-	movl EFLAGS(%ebx), %ecx	# and this is cs..
-	movl %eax,EFLAGS(%ebx)	#
-	movl %edx,EIP(%ebx)	# Now we move them to their "normal" places
-	movl %ecx,CS(%ebx)	#
+	movl EIP(%ebp), %eax	# due to call gates, this is eflags, not eip..
+	movl CS(%ebp), %edx	# this is eip..
+	movl EFLAGS(%ebp), %ecx	# and this is cs..
+	movl %eax,EFLAGS(%ebp)	#
+	movl %edx,EIP(%ebp)	# Now we move them to their "normal" places
+	movl %ecx,CS(%ebp)	#
 
 	#
 	# Call gates don't clear TF and NT in eflags like
@@ -166,8 +166,8 @@ do_lcall:
 	pushl %eax
 	popfl
 
-	andl $-8192, %ebx	# GET_THREAD_INFO
-	movl TI_EXEC_DOMAIN(%ebx), %edx	# Get the execution domain
+	andl $-8192, %ebp	# GET_THREAD_INFO
+	movl TI_EXEC_DOMAIN(%ebp), %edx	# Get the execution domain
 	call *4(%edx)		# Call the lcall7 handler for the domain
 	addl $4, %esp
 	popl %eax
@@ -178,8 +178,8 @@ ENTRY(lcall27)
 				# gates, which has to be cleaned up later..
 	pushl %eax
 	SAVE_ALL
-	movl %esp, %ebx
-	pushl %ebx
+	movl %esp, %ebp
+	pushl %ebp
 	pushl $0x27
 	jmp do_lcall
 
@@ -187,7 +187,7 @@ ENTRY(lcall27)
 ENTRY(ret_from_fork)
 	# NOTE: this function takes a parameter but it's unused on x86.
 	call schedule_tail
-	GET_THREAD_INFO(%ebx)
+	GET_THREAD_INFO(%ebp)
 	jmp syscall_exit
 
 /*
@@ -202,7 +202,7 @@ ENTRY(ret_from_fork)
 ret_from_exception:
 	preempt_stop
 ret_from_intr:
-	GET_THREAD_INFO(%ebx)
+	GET_THREAD_INFO(%ebp)
 	movl EFLAGS(%esp), %eax		# mix EFLAGS and CS
 	movb CS(%esp), %al
 	testl $(VM_MASK | 3), %eax
@@ -211,7 +211,7 @@ ENTRY(resume_userspace)
  	cli				# make sure we don't miss an interrupt
 					# setting need_resched or sigpending
 					# between sampling and the iret
-	movl TI_FLAGS(%ebx), %ecx
+	movl TI_FLAGS(%ebp), %ecx
 	andl $_TIF_WORK_MASK, %ecx	# is there any work to be done on
 					# int/exception return?
 	jne work_pending
@@ -219,18 +219,18 @@ ENTRY(resume_userspace)
 
 #ifdef CONFIG_PREEMPT
 ENTRY(resume_kernel)
-	cmpl $0,TI_PRE_COUNT(%ebx)	# non-zero preempt_count ?
+	cmpl $0,TI_PRE_COUNT(%ebp)	# non-zero preempt_count ?
 	jnz restore_all
 need_resched:
-	movl TI_FLAGS(%ebx), %ecx	# need_resched set ?
+	movl TI_FLAGS(%ebp), %ecx	# need_resched set ?
 	testb $_TIF_NEED_RESCHED, %cl
 	jz restore_all
 	testl $IF_MASK,EFLAGS(%esp)     # interrupts off (execption path) ?
 	jz restore_all
-	movl $PREEMPT_ACTIVE,TI_PRE_COUNT(%ebx)
+	movl $PREEMPT_ACTIVE,TI_PRE_COUNT(%ebp)
 	sti
 	call schedule
-	movl $0,TI_PRE_COUNT(%ebx) 
+	movl $0,TI_PRE_COUNT(%ebp)
 	cli
 	jmp need_resched
 #endif
@@ -262,21 +262,21 @@ ENTRY(sysenter_entry)
 
 	pushl %eax
 	SAVE_ALL
-	GET_THREAD_INFO(%ebx)
+	GET_THREAD_INFO(%ebp)
 	cmpl $(NR_syscalls), %eax
 	jae syscall_badsys
 
-	testb $_TIF_SYSCALL_TRACE,TI_FLAGS(%ebx)
+	testb $_TIF_SYSCALL_TRACE,TI_FLAGS(%ebp)
 	jnz syscall_trace_entry
 	call *sys_call_table(,%eax,4)
 	movl %eax,EAX(%esp)
 	cli
-	movl TI_FLAGS(%ebx), %ecx
+	movl TI_FLAGS(%ebp), %ecx
 	testw $_TIF_ALLWORK_MASK, %cx
 	jne syscall_exit_work
	RESTORE_INT_REGS
@@ -286,11 +286,11 @@ ENTRY(sysenter_entry)
 ENTRY(system_call)
 	pushl %eax			# save orig_eax
 	SAVE_ALL
-	GET_THREAD_INFO(%ebx)
+	GET_THREAD_INFO(%ebp)
 	cmpl $(NR_syscalls), %eax
 	jae syscall_badsys
 					# system call tracing in operation
-	testb $_TIF_SYSCALL_TRACE,TI_FLAGS(%ebx)
+	testb $_TIF_SYSCALL_TRACE,TI_FLAGS(%ebp)
 	jnz syscall_trace_entry
 syscall_call:
 	call *sys_call_table(,%eax,4)
@@ -299,7 +299,7 @@ syscall_exit:
 	cli				# make sure we don't miss an interrupt
 					# setting need_resched or sigpending
 					# between sampling and the iret
-	movl TI_FLAGS(%ebx), %ecx
+	movl TI_FLAGS(%ebp), %ecx
 	testw $_TIF_ALLWORK_MASK, %cx	# current->work
 	jne syscall_exit_work
 restore_all:
@@ -315,7 +315,7 @@ work_resched:
 	cli				# make sure we don't miss an interrupt
 					# setting need_resched or sigpending
 					# between sampling and the iret
-	movl TI_FLAGS(%ebx), %ecx
+	movl TI_FLAGS(%ebp), %ecx
 	andl $_TIF_WORK_MASK, %ecx	# is there any work to be done other
 					# than syscall tracing?
 	jz restore_all
@@ -370,7 +370,7 @@ syscall_exit_work:
 syscall_fault:
 	pushl %eax			# save orig_eax
 	SAVE_ALL
-	GET_THREAD_INFO(%ebx)
+	GET_THREAD_INFO(%ebp)
 	movl $-EFAULT,EAX(%esp)
 	jmp resume_userspace
 

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Re: [PATCH] Use %ebp rather than %ebx for thread_info pointer

[Richard B. Johnson]

On Thu, 9 Jan 2003, Luca Barbieri wrote:

> This patch changes assembly code that accesses thread_info to use %ebp
> rather than %ebx. 
> 
> This allows me to take advantage of the fact that %ebp is restored by
> user mode in the sysenter register pop removal patch.

If you use EBP as an index register, i.e., "movl (%ebp), %eax", it
will be relative to the SS, not ES or DS. Is this what you want?

Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
Why is the government concerned about the lunatic fringe? Think about it.

Re: [PATCH] Use %ebp rather than %ebx for thread_info pointer

[Linus Torvalds]

On Thu, 9 Jan 2003, Luca Barbieri wrote:
>
> This patch changes assembly code that accesses thread_info to use %ebp
> rather than %ebx. 
> 
> This allows me to take advantage of the fact that %ebp is restored by
> user mode in the sysenter register pop removal patch.

Hmm.. Did you check what fork/execve/vm86 do? I know at least the vm86()  
stuff sets up %ebx before calling the asm functions in entry.S, I bet 
those need to be changed to use %ebp too with something like this.

		Linus

Re: [PATCH] Use %ebp rather than %ebx for thread_info pointer

[Linus Torvalds]

On Thu, 9 Jan 2003, Richard B. Johnson wrote:
> 
> If you use EBP as an index register, i.e., "movl (%ebp), %eax", it
> will be relative to the SS, not ES or DS. Is this what you want?

That's fine, both SS and DS are 32-bit flat segments everywhere in the
kernel (they have different descriptors - __KERNEL_DS vs __USER_DS, but
they do the same thing)

		Linus

Re: [PATCH] Use %ebp rather than %ebx for thread_info pointer

[Luca Barbieri]

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> Hmm.. Did you check what fork
It only seems to use ret_from_fork, that sets up %ebp on its own.
> execve
Doesn't seem to use any function in entry.S
> vm86 do? I know at least the vm86()  
> stuff sets up %ebx before calling the asm functions in entry.S, I bet 
> those need to be changed to use %ebp too with something like this.

Right. This should fix it (compiles but untested):
--- arch/i386/kernel/vm86.c~    2003-01-02 04:21:07.000000000 +0100
+++ arch/i386/kernel/vm86.c     2003-01-09 21:24:58.000000000 +0100
@@ -298,9 +298,10 @@
        __asm__ __volatile__(
                "xorl %%eax,%%eax; movl %%eax,%%fs; movl %%eax,%%gs\n\t"
                "movl %0,%%esp\n\t"
+               "movl %1,%%ebp\n\t"
                "jmp resume_userspace"
                : /* no outputs */
-               :"r" (&info->regs), "b" (tsk->thread_info) : "ax");
+               :"r" (&info->regs), "r" (tsk->thread_info) : "ax");
        /* we never return here */
 }
  
@@ -311,8 +312,9 @@
        regs32 = save_v86_state(regs16);
        regs32->eax = retval;
        __asm__ __volatile__("movl %0,%%esp\n\t"
+               "movl %1,%%ebp\n\t"
                "jmp resume_userspace"
-               : : "r" (regs32), "b" (current_thread_info()));
+               : : "r" (regs32), "r" (current_thread_info()));
 }
  
 static inline void set_IF(struct kernel_vm86_regs * regs)


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