make seccomp zerocost in schedule

This follows a suggestion from Chuck Ebbert on how to make seccomp
absolutely zerocost in schedule too.  The only remaining footprint of
seccomp is in terms of the bzImage size that becomes a few bytes (perhaps
even a few kbytes) larger, measure it if you care in the embedded.

Signed-off-by: Andrea Arcangeli <andrea@cpushare.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c
index 06dfa65..6c49acb 100644 (file)
--- a/arch/i386/kernel/process.c
+++ b/arch/i386/kernel/process.c
@@ -538,8 +538,31 @@ int dump_task_regs(struct task_struct *tsk, elf_gregset_t *regs)
        return 1;
 }
 
-static noinline void __switch_to_xtra(struct task_struct *next_p,
-                                   struct tss_struct *tss)
+#ifdef CONFIG_SECCOMP
+void hard_disable_TSC(void)
+{
+       write_cr4(read_cr4() | X86_CR4_TSD);
+}
+void disable_TSC(void)
+{
+       preempt_disable();
+       if (!test_and_set_thread_flag(TIF_NOTSC))
+               /*
+                * Must flip the CPU state synchronously with
+                * TIF_NOTSC in the current running context.
+                */
+               hard_disable_TSC();
+       preempt_enable();
+}
+void hard_enable_TSC(void)
+{
+       write_cr4(read_cr4() & ~X86_CR4_TSD);
+}
+#endif /* CONFIG_SECCOMP */
+
+static noinline void
+__switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
+                struct tss_struct *tss)
 {
        struct thread_struct *next;
 
@@ -555,6 +578,17 @@ static noinline void __switch_to_xtra(struct task_struct *next_p,
                set_debugreg(next->debugreg[7], 7);
        }
 
+#ifdef CONFIG_SECCOMP
+       if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
+           test_tsk_thread_flag(next_p, TIF_NOTSC)) {
+               /* prev and next are different */
+               if (test_tsk_thread_flag(next_p, TIF_NOTSC))
+                       hard_disable_TSC();
+               else
+                       hard_enable_TSC();
+       }
+#endif
+
        if (!test_tsk_thread_flag(next_p, TIF_IO_BITMAP)) {
                /*
                 * Disable the bitmap via an invalid offset. We still cache
@@ -586,33 +620,6 @@ static noinline void __switch_to_xtra(struct task_struct *next_p,
 }
 
 /*
- * This function selects if the context switch from prev to next
- * has to tweak the TSC disable bit in the cr4.
- */
-static inline void disable_tsc(struct task_struct *prev_p,
-                              struct task_struct *next_p)
-{
-       struct thread_info *prev, *next;
-
-       /*
-        * gcc should eliminate the ->thread_info dereference if
-        * has_secure_computing returns 0 at compile time (SECCOMP=n).
-        */
-       prev = task_thread_info(prev_p);
-       next = task_thread_info(next_p);
-
-       if (has_secure_computing(prev) || has_secure_computing(next)) {
-               /* slow path here */
-               if (has_secure_computing(prev) &&
-                   !has_secure_computing(next)) {
-                       write_cr4(read_cr4() & ~X86_CR4_TSD);
-               } else if (!has_secure_computing(prev) &&
-                          has_secure_computing(next))
-                       write_cr4(read_cr4() | X86_CR4_TSD);
-       }
-}
-
-/*
  *     switch_to(x,yn) should switch tasks from x to y.
  *
  * We fsave/fwait so that an exception goes off at the right time
@@ -689,11 +696,9 @@ struct task_struct fastcall * __switch_to(struct task_struct *prev_p, struct tas
        /*
         * Now maybe handle debug registers and/or IO bitmaps
         */
-       if (unlikely((task_thread_info(next_p)->flags & _TIF_WORK_CTXSW)
-           || test_tsk_thread_flag(prev_p, TIF_IO_BITMAP)))
-               __switch_to_xtra(next_p, tss);
-
-       disable_tsc(prev_p, next_p);
+       if (unlikely(task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV ||
+                    task_thread_info(next_p)->flags & _TIF_WORK_CTXSW_NEXT))
+               __switch_to_xtra(prev_p, next_p, tss);
 
        /*
         * Leave lazy mode, flushing any hypercalls made here.

diff --git a/include/asm-i386/processor.h b/include/asm-i386/processor.h
index 94e0c14..422cffe 100644 (file)
--- a/include/asm-i386/processor.h
+++ b/include/asm-i386/processor.h
@@ -228,6 +228,10 @@ extern int bootloader_type;
 
 #define HAVE_ARCH_PICK_MMAP_LAYOUT
 
+extern void hard_disable_TSC(void);
+extern void disable_TSC(void);
+extern void hard_enable_TSC(void);
+
 /*
  * Size of io_bitmap.
  */

diff --git a/include/asm-i386/thread_info.h b/include/asm-i386/thread_info.h
index 4cb0f91..54424e0 100644 (file)
--- a/include/asm-i386/thread_info.h
+++ b/include/asm-i386/thread_info.h
@@ -137,6 +137,7 @@ static inline struct thread_info *current_thread_info(void)
 #define TIF_DEBUG              17      /* uses debug registers */
 #define TIF_IO_BITMAP          18      /* uses I/O bitmap */
 #define TIF_FREEZE             19      /* is freezing for suspend */
+#define TIF_NOTSC              20      /* TSC is not accessible in userland */
 
 #define _TIF_SYSCALL_TRACE     (1<<TIF_SYSCALL_TRACE)
 #define _TIF_NOTIFY_RESUME     (1<<TIF_NOTIFY_RESUME)
@@ -151,6 +152,7 @@ static inline struct thread_info *current_thread_info(void)
 #define _TIF_DEBUG             (1<<TIF_DEBUG)
 #define _TIF_IO_BITMAP         (1<<TIF_IO_BITMAP)
 #define _TIF_FREEZE            (1<<TIF_FREEZE)
+#define _TIF_NOTSC             (1<<TIF_NOTSC)
 
 /* work to do on interrupt/exception return */
 #define _TIF_WORK_MASK \
@@ -160,7 +162,8 @@ static inline struct thread_info *current_thread_info(void)
 #define _TIF_ALLWORK_MASK      (0x0000FFFF & ~_TIF_SECCOMP)
 
 /* flags to check in __switch_to() */
-#define _TIF_WORK_CTXSW (_TIF_DEBUG|_TIF_IO_BITMAP)
+#define _TIF_WORK_CTXSW_NEXT (_TIF_IO_BITMAP | _TIF_NOTSC | _TIF_DEBUG)
+#define _TIF_WORK_CTXSW_PREV (_TIF_IO_BITMAP | _TIF_NOTSC)
 
 /*
  * Thread-synchronous status.

diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index d708974..262a8dc 100644 (file)
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -16,11 +16,6 @@ static inline void secure_computing(int this_syscall)
                __secure_computing(this_syscall);
 }
 
-static inline int has_secure_computing(struct thread_info *ti)
-{
-       return unlikely(test_ti_thread_flag(ti, TIF_SECCOMP));
-}
-
 extern long prctl_get_seccomp(void);
 extern long prctl_set_seccomp(unsigned long);
 
@@ -29,11 +24,6 @@ extern long prctl_set_seccomp(unsigned long);
 typedef struct { } seccomp_t;
 
 #define secure_computing(x) do { } while (0)
-/* static inline to preserve typechecking */
-static inline int has_secure_computing(struct thread_info *ti)
-{
-       return 0;
-}
 
 static inline long prctl_get_seccomp(void)
 {

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 1dfa8a5..ad64fcb 100644 (file)
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -74,6 +74,9 @@ long prctl_set_seccomp(unsigned long seccomp_mode)
        if (seccomp_mode && seccomp_mode <= NR_SECCOMP_MODES) {
                current->seccomp.mode = seccomp_mode;
                set_thread_flag(TIF_SECCOMP);
+#ifdef TIF_NOTSC
+               disable_TSC();
+#endif
                ret = 0;
        }