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Subject[PATCH 2/5] uprobes: introduce uprobe_switch_to()
Introduce uprobe_switch_to(), it is called by switch_to() paths if
the "next" task is going to execute the xol insn.

Currently we use TIF_SINGLESTEP (added to _TIF_WORK_CTXSW_NEXT) to
detect this case in __switch_to_xtra() and call uprobe_switch_to(),
may be we can add another flag.

uprobe_switch_to() verifies that this task is actually UTASK_SSTEP
and X86_EFLAGS_TF is set.

Finally uprobe_switch_to() does set_xol_ip(). Currently this is not
needed, but this means that set_xol_ip() is called every time the
UTASK_SSTEP task migrates to another CPU.

To ensure set_xol_ip() can't race with itself we add preempt_disable()
into another caller, uprobe_notify_resume().

Note! this patch assumes we can trust X86_EFLAGS_TF. I mean, afaiu
even if the single-stepping insn races with irq/exception, this flag
will be cleared if and only if this instruction was already executed.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
arch/x86/include/asm/thread_info.h | 4 ++++
arch/x86/kernel/process.c | 6 ++++++
arch/x86/kernel/uprobes.c | 14 ++++++++++++++
include/linux/uprobes.h | 1 +
kernel/uprobes.c | 2 ++
5 files changed, 27 insertions(+), 0 deletions(-)
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index aeb3e04..af711a1 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -150,7 +150,11 @@ struct thread_info {
(_TIF_IO_BITMAP|_TIF_NOTSC|_TIF_BLOCKSTEP)

#define _TIF_WORK_CTXSW_PREV (_TIF_WORK_CTXSW|_TIF_USER_RETURN_NOTIFY)
+#ifdef CONFIG_UPROBES
+#define _TIF_WORK_CTXSW_NEXT (_TIF_WORK_CTXSW|_TIF_DEBUG|_TIF_SINGLESTEP)
+#else
#define _TIF_WORK_CTXSW_NEXT (_TIF_WORK_CTXSW|_TIF_DEBUG)
+#endif

#define PREEMPT_ACTIVE 0x10000000

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index b9b3b1a..233bf20 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -229,6 +229,12 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
*/
memset(tss->io_bitmap, 0xff, prev->io_bitmap_max);
}
+
+#ifdef CONFIG_UPROBES
+ if (test_tsk_thread_flag(next_p, TIF_SINGLESTEP))
+ uprobe_switch_to(next_p);
+#endif
+
propagate_user_return_notify(prev_p, next_p);
}

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index cd086be..4140137 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -452,6 +452,20 @@ int pre_xol(struct uprobe *uprobe, struct pt_regs *regs)
}
#endif

+void uprobe_switch_to(struct task_struct *curr)
+{
+ struct uprobe_task *utask = curr->utask;
+ struct pt_regs *regs = task_pt_regs(curr);
+
+ if (!utask || utask->state != UTASK_SSTEP)
+ return;
+
+ if (!(regs->flags & X86_EFLAGS_TF))
+ return;
+
+ set_xol_ip(regs);
+}
+
/*
* Called by post_xol() to adjust the return address pushed by a call
* instruction executed out of line.
diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index c9ff67a..d590d66 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -141,6 +141,7 @@ extern void uprobe_notify_resume(struct pt_regs *regs);
extern bool uprobe_deny_signal(void);
extern bool __weak can_skip_xol(struct pt_regs *regs, struct uprobe *u);
extern void __weak set_xol_ip(struct pt_regs *regs);
+extern void uprobe_switch_to(struct task_struct *);
#else /* CONFIG_UPROBES is not defined */
static inline int register_uprobe(struct inode *inode, loff_t offset,
struct uprobe_consumer *consumer)
diff --git a/kernel/uprobes.c b/kernel/uprobes.c
index b596432..9c509dc 100644
--- a/kernel/uprobes.c
+++ b/kernel/uprobes.c
@@ -1399,8 +1399,10 @@ void uprobe_notify_resume(struct pt_regs *regs)
goto cleanup_ret;

user_enable_single_step(current);
+ preempt_disable();
utask->state = UTASK_SSTEP;
set_xol_ip(regs);
+ preempt_enable();
} else {
u = utask->active_uprobe;
if (utask->state == UTASK_SSTEP_ACK)
--
1.5.5.1



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