Merge branch 'linus' into tracing/hw-breakpoints

[~andy/linux] / arch / x86 / kernel / process_32.c

diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 59f4524984afacd10b456dcdca4abbc2237d7ecb..00a8fe4c58bbc43bfde0bb9f27fdfc56a9c33967 100644 (file)
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -58,6 +58,8 @@
 #include <asm/idle.h>
 #include <asm/syscalls.h>
 #include <asm/ds.h>
+#include <asm/debugreg.h>
+#include <asm/hw_breakpoint.h>
 
 asmlinkage void ret_from_fork(void) __asm__("ret_from_fork");
 
@@ -262,7 +264,13 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
 
        task_user_gs(p) = get_user_gs(regs);
 
+       p->thread.io_bitmap_ptr = NULL;
        tsk = current;
+       err = -ENOMEM;
+       if (unlikely(test_tsk_thread_flag(tsk, TIF_DEBUG)))
+               if (copy_thread_hw_breakpoint(tsk, p, clone_flags))
+                       goto out;
+
        if (unlikely(test_tsk_thread_flag(tsk, TIF_IO_BITMAP))) {
                p->thread.io_bitmap_ptr = kmemdup(tsk->thread.io_bitmap_ptr,
                                                IO_BITMAP_BYTES, GFP_KERNEL);
@@ -282,10 +290,13 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
                err = do_set_thread_area(p, -1,
                        (struct user_desc __user *)childregs->si, 0);
 
+out:
        if (err && p->thread.io_bitmap_ptr) {
                kfree(p->thread.io_bitmap_ptr);
                p->thread.io_bitmap_max = 0;
        }
+       if (err)
+               flush_thread_hw_breakpoint(p);
 
        clear_tsk_thread_flag(p, TIF_DS_AREA_MSR);
        p->thread.ds_ctx = NULL;
@@ -424,6 +435,23 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
                lazy_load_gs(next->gs);
 
        percpu_write(current_task, next_p);
+       /*
+        * There's a problem with moving the arch_install_thread_hw_breakpoint()
+        * call before current is updated.  Suppose a kernel breakpoint is
+        * triggered in between the two, the hw-breakpoint handler will see that
+        * the 'current' task does not have TIF_DEBUG flag set and will think it
+        * is leftover from an old task (lazy switching) and will erase it. Then
+        * until the next context switch, no user-breakpoints will be installed.
+        *
+        * The real problem is that it's impossible to update both current and
+        * physical debug registers at the same instant, so there will always be
+        * a window in which they disagree and a breakpoint might get triggered.
+        * Since we use lazy switching, we are forced to assume that a
+        * disagreement means that current is correct and the exception is due
+        * to lazy debug register switching.
+        */
+       if (unlikely(test_tsk_thread_flag(next_p, TIF_DEBUG)))
+               arch_install_thread_hw_breakpoint(next_p);
 
        return prev_p;
 }