[C/R ARM][PATCH 1/3] ARM: Rudimentary syscall interfaces

[Christoffer Dall]

On Wed, Mar 24, 2010 at 4:53 PM, Oren Laadan <orenl at cs.columbia.edu> wrote:
>
>
> Matt Helsley wrote:
>>
>> On Wed, Mar 24, 2010 at 12:57:46AM -0400, Oren Laadan wrote:
>>>
>>> On Tue, 23 Mar 2010, Matt Helsley wrote:
>>>
>>>> On Tue, Mar 23, 2010 at 08:53:42PM +0000, Russell King - ARM Linux
>>>> wrote:
>>>>>
>>>>> On Sun, Mar 21, 2010 at 09:06:03PM -0400, Christoffer Dall wrote:
>>>>>>
>>>>>> This small commit introduces a global state of system calls for ARM
>>>>>> making it possible for a debugger or checkpointing to gain information
>>>>>> about another process' state with respect to system calls.
>>>>>
>>>>> I don't particularly like the idea that we always store the syscall
>>>>> number to memory for every system call, whether the stored version is
>>>>> used or not.
>>>>>
>>>>> Since ARM caches are generally not write allocate, this means mostly
>>>>> write-only variables can have a higher than expected expense.
>>>>>
>>>>> Is there not some thread flag which can be checked to see if we need to
>>>>> store the syscall number?
>>>>
>>>> Perhaps before we freeze the task we can save the syscall number on ARM.
>>>> The patches suggest that the signal delivery path -- which the freezer
>>>> utilizes -- has the syscall number already.
>
> Actually, the signal path doesn't have the syscall number, it has
> a binary "in syscall" value.
>

Well, this could be changed to pass the syscall number through
registers along to try_to_freeze without any mentionable performance
hit.

>>>>
>>>> Should work since the threads must be frozen first anyway.
>>>
>>> I like the idea.
>>>
>>> However, would it also work for those cases when the freezing does not
>>> occur from the signal delivery path - e.g. for vfork and ptraced tasks ?
>>
>> We could just as easily set it before the vfork uninterruptible
>> completion.
>> ptracing I'd don't know about though.
>>
>
> vfork() uses freezer_do_not_count() to tell the freezer that it's
> effectively frozen. It's also used by drivers/char/apm-emulation.c
>
> Looking at calls to ptrace_notify(), ptrace_stop() and ptace_event(),
> there are several places where a ptraced task can stop with TASK_TRACED
> (which is good enough for the freezer), outside the signal handling
> path.
>
> This means that recording the syscall number for all these cases is
> going to be tedious and intrusive.
>
> I prefer to somehow figure out the syscall from the task's state or
> pt_regs, or by (re)using the same assembly code that already does that.

Re-using the assembly code or factoring it out so that it can be used
from multiple places doesn't seem very pleasing to me, as the assembly
code is in the critical path and written specifically for the context
of a process entering the kernel. Please correct me if I'm wrong.

I imagine simply a function in C, more or less re-implementing the
logic that's already in entry-common.S, might do the trick. I wouldn't
worry much about the performance in this case as it will not be used
often. The following _untested_ snippet illustrates my idea:

---
 arch/arm/include/asm/syscall.h |   93 +++++++++++++++++++++++++++++++++++++++-
 1 files changed, 92 insertions(+), 1 deletions(-)

diff --git a/arch/arm/include/asm/syscall.h b/arch/arm/include/asm/syscall.h
index 3b3248f..a7f2615 100644
--- a/arch/arm/include/asm/syscall.h
+++ b/arch/arm/include/asm/syscall.h
@@ -10,10 +10,101 @@
 #ifndef _ASM_ARM_SYSCALLS_H
 #define _ASM_ARM_SYSCALLS_H

+static inline int get_swi_instruction(struct task_struct *task,
+				      struct pt_regs *regs,
+				      unsigned long *instr)
+{
+	struct page *page = NULL;
+	unsigned long instr_addr;
+	unsigned long *ptr;
+	int ret;
+
+	instr_addr = regs->ARM_pc - 4;
+
+	down_read(&task->mm->mmap_sem);
+	ret = get_user_pages(task, task->mm, instr_addr,
+			     1, 0, 0, &page, NULL);
+	up_read(&task->mm->mmap_sem);
+
+	if (ret < 0)
+		return ret;
+
+	ptr = (unsigned long *)kmap_atomic(page, KM_USER1);
+	memcpy(instr,
+	       ptr + (instr_addr >> PAGE_SHIFT),
+	       sizeof(unsigned long));
+	kunmap_atomic(ptr, KM_USER1);
+
+	page_cache_release(page);
+
+	return 0;
+}
+
+static inline int __syscall_get_nr(struct task_struct *task,
+				   struct pt_regs *regs)
+{
+	int ret;
+	int scno;
+	unsigned long instr;
+	bool config_oabi = false;
+	bool config_aeabi = false;
+	bool config_arm_thumb = false;
+	bool config_cpu_endian_be8 = false;
+
+#ifdef CONFIG_OABI_COMPAT
+	config_oabi = true;
+#endif
+#ifdef CONFIG_AEABI
+	config_aeabi = true;
+#endif
+#ifdef CONFIG_ARM_THUMB
+	config_arm_thumb = true;
+#endif
+#ifdef CONFIG_CPU_ENDIAN_BE8
+	config_cpu_endian_be8 = true;
+#endif
+#ifdef CONFIG_CPU_ARM710
+	return -1;
+#endif
+
+	if (config_aeabi && !config_oabi) {
+		/* Pure EABI */
+		return regs->ARM_r7;
+	} else if (config_oabi) {
+		if (config_arm_thumb && (regs->ARM_cpsr & PSR_T_BIT))
+			return -1;
+
+		ret = get_swi_instruction(task, regs, &instr);
+		if (ret < 0)
+			return -1;
+
+		if (config_cpu_endian_be8)
+			asm ("rev %[out], %[in]": [out] "=r" (instr):
+						: [in] "r" (instr));
+
+		if ((instr & 0x00ffffff) == 0)
+			return regs->ARM_r7; /* EABI call */
+		else
+			return (instr & 0x00ffffff) | __NR_OABI_SYSCALL_BASE;
+	} else {
+		 /* Legacy ABI only */
+		if (config_arm_thumb && (regs->ARM_cpsr & PSR_T_BIT)) {
+			/* Thumb mode ABI */
+			scno = regs->ARM_r7 + __NR_SYSCALL_BASE;
+		} else {
+			ret = get_swi_instruction(task, regs, &instr);
+			if (ret < 0)
+				return -1;
+			scno = instr;
+		}
+		return scno & 0x00ffffff;
+	}
+}
+
 static inline int syscall_get_nr(struct task_struct *task,
 				 struct pt_regs *regs)
 {
-	return (int)(task_thread_info(task)->syscall);
+	return __syscall_get_nr(task, regs);
 }

 static inline long syscall_get_return_value(struct task_struct *task,
-- 
1.5.6.5