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From:   Oliver Swede <oli.swede@arm.com>
To:     catalin.marinas@arm.com, will@kernel.org
Cc:     robin.murphy@arm.com, linux-arm-kernel@lists.indradead.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH v5 02/14] arm64: kprobes: Drop open-coded exception fixup
Date:   Mon, 14 Sep 2020 15:09:46 +0000
Message-Id: <20200914150958.2200-3-oli.swede@arm.com>
In-Reply-To: <20200914150958.2200-1-oli.swede@arm.com>
References: <20200914150958.2200-1-oli.swede@arm.com>
Sender: linux-kernel-owner@vger.kernel.org
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From: Robin Murphy <robin.murphy@arm.com>

The short-circuit call to fixup_exception() from kprobe_fault_handler()
poses a problem now that the former wants to consume the fault address
too, since the common kprobes API offers us no way to pass it through.
Fortunately, however, it works out to be unnecessary:

- uaccess instructions themselves are not probeable, so at most we
  should only ever expect to take a fixable fault from the pre or post
  handlers.
- the pre and post handler run with preemption disabled, thus for any
  fault they may cause, an unhandled return from kprobe_page_fault()
  will proceed directly to __do_kernel_fault() thanks to the
  faulthandler_disabled() check.
- __do_kernel_fault() will immediately call fixup_exception() unless
  we're in an EL1 instruction abort, and if we've somehow taken one of
  those on what we think is the middle of a uaccess routine, then the
  world is already very on fire.

Thus we can reasonably drop the call from kprobe_fault_handler() and
leave uaccess fixups to the regular flow.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Oliver Swede <oli.swede@arm.com>
---
 arch/arm64/kernel/probes/kprobes.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index 5290f17a4d80..c54c8252b32f 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -328,13 +328,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
 		 */
 		if (cur->fault_handler && cur->fault_handler(cur, regs, fsr))
 			return 1;
-
-		/*
-		 * In case the user-specified fault handler returned
-		 * zero, try to fix up.
-		 */
-		if (fixup_exception(regs))
-			return 1;
 	}
 	return 0;
 }
-- 
2.17.1