linux-kernel - [PATCH] ARC: make sure instruction_pointer() returns unsigned value

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Date:	Mon, 13 Jul 2015 10:25:17 +0300
From:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:	Vineet.Gupta1@...opsys.com
Cc:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	arc-linux-dev@...opsys.com, stable@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] ARC: make sure instruction_pointer() returns unsigned value

Currently instruction_pointer() returns pt_regs->ret and so return value
is of type "long", which implicitly stands for "signed long".

While that's perfectly fine when dealing with 32-bit values if return
value of instruction_pointer() gets assigned to 64-bit variable sign
extension may happen.

And at least in one real use-case it happens already.
In perf_prepare_sample() return value of perf_instruction_pointer()
(which is an alias to instruction_pointer() in case of ARC) is assigned
to (struct perf_sample_data)->ip (which type is "u64").

And what we see if instuction pointer points to user-space application
that in case of ARC lays below 0x8000_0000 "ip" gets set properly with
leading 32 zeros. But if instruction pointer points to kernel address
space that starts from 0x8000_0000 then "ip" is set with 32 leadig
"f"-s. I.e. id instruction_pointer() returns 0x8100_0000, "ip" will be
assigned with 0xffff_ffff__8100_0000. Which is obviously wrong.

In particular that issuse broke output of perf, because perf was unable
to associate addresses like 0xffff_ffff__8100_0000 with anything from
/proc/kallsyms.

That's what we used to see:
 ----------->8----------
  6.27%  ls       [unknown]                [k] 0xffffffff8046c5cc
  2.96%  ls       libuClibc-0.9.34-git.so  [.] memcpy
  2.25%  ls       libuClibc-0.9.34-git.so  [.] memset
  1.66%  ls       [unknown]                [k] 0xffffffff80666536
  1.54%  ls       libuClibc-0.9.34-git.so  [.] 0x000224d6
  1.18%  ls       libuClibc-0.9.34-git.so  [.] 0x00022472
 ----------->8----------

With that change perf output looks much better now:
 ----------->8----------
  8.21%  ls       [kernel.kallsyms]        [k] memset
  3.52%  ls       libuClibc-0.9.34-git.so  [.] memcpy
  2.11%  ls       libuClibc-0.9.34-git.so  [.] malloc
  1.88%  ls       libuClibc-0.9.34-git.so  [.] memset
  1.64%  ls       [kernel.kallsyms]        [k] _raw_spin_unlock_irqrestore
  1.41%  ls       [kernel.kallsyms]        [k] __d_lookup_rcu
 ----------->8----------

Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
Cc: arc-linux-dev@...opsys.com
Cc: stable@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
---
 arch/arc/include/asm/ptrace.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arc/include/asm/ptrace.h b/arch/arc/include/asm/ptrace.h
index 9175597..91694ec 100644
--- a/arch/arc/include/asm/ptrace.h
+++ b/arch/arc/include/asm/ptrace.h
@@ -106,7 +106,7 @@ struct callee_regs {
 	long r25, r24, r23, r22, r21, r20, r19, r18, r17, r16, r15, r14, r13;
 };

-#define instruction_pointer(regs)	((regs)->ret)
+#define instruction_pointer(regs)	(unsigned long)((regs)->ret)
 #define profile_pc(regs)		instruction_pointer(regs)

 /* return 1 if user mode or 0 if kernel mode */
-- 
2.4.3

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