Re: [PATCH] tools/perf: fix the word selected in find_*_bit

On Thursday 16 June 2016 02:41 AM, Yury Norov wrote:

On Wed, Jun 15, 2016 at 10:51:27PM +0300, Yury Norov wrote:

quoted

Hi Madhavan,

On Wed, Jun 15, 2016 at 05:12:53PM +0530, Madhavan Srinivasan wrote:

quoted

When decoding the perf_regs mask in regs_dump__printf(),
we loop through the mask using find_first_bit and find_next_bit functions.
And mask is of type "u64". But "u64" is send as a "unsigned long *" to
lib functions along with sizeof().

While the exisitng code works fine in most of the case, when using a 32bit perf
on a 64bit kernel (Big Endian), we end reading the wrong word. In find_first_bit(),
one word at a time (based on BITS_PER_LONG) is loaded and
checked for any bit set. In 32bit BE userspace,
BITS_PER_LONG turns out to be 32, and for a mask value of
"0x00000000000000ff", find_first_bit will return 32, instead of 0.
Reason for this is that, value in the word0 is all zeros and value
in word1 is 0xff. Ideally, second word in the mask should be loaded
and searched. Patch swaps the word to look incase of 32bit BE.

I think this is not a problem of find_bit() at all. You have wrong
typecast as the source of problem (tools/perf/util/session.c"):

940 static void regs_dump__printf(u64 mask, u64 *regs)
941 {
942         unsigned rid, i = 0;
943 
944         for_each_set_bit(rid, (unsigned long *) &mask, sizeof(mask) * 8) {
                                          ^^^^ Here ^^^^
945                 u64 val = regs[i++];
946 
947                 printf(".... %-5s 0x%" PRIx64 "\n",
948                        perf_reg_name(rid), val);
949         }
950 }

But for some reason you change correct find_bit()...

Though proper fix is like this for me:

static void regs_dump__printf(u64 mask, u64 *regs)
{
        unsigned rid, i = 0;
        unsigned long _mask[sizeof(mask)/sizeof(unsigned long)];

        _mask[0] = mask & ULONG_MAX;
        if (sizeof(mask) > sizeof(unsigned long))
                _mask[1] = mask >> BITS_PER_LONG;

        for_each_set_bit(rid, _mask, sizeof(mask) * BITS_PER_BYTE) {
                u64 val = regs[i++];

                printf(".... %-5s 0x%" PRIx64 "\n",
                       perf_reg_name(rid), val);
        }
}

Maybe there already is some macro doing the conversion for you...

yes it is, cpu_to_le64() is what you want

no wait, on second look, cpu_to_le64() is not right.
Because we will end up swapping within 32bit.
But what you suggested looks to be fine. I can repost
this with one minor tweak, right shift with 32 instead
of BITS_PER_LONG (since I see compiler errors in 64bit).

Maddy

quoted

Yury.

quoted

Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <redacted>
Cc: David Ahern <redacted>
Cc: George Spelvin <redacted>
Cc: Jiri Olsa <redacted>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Wang Nan <redacted>
Cc: Yury Norov <yury.norov@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <redacted>
---
 tools/lib/find_bit.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/tools/lib/find_bit.c b/tools/lib/find_bit.c
index 9122a9e80046..996b3e04324f 100644
--- a/tools/lib/find_bit.c
+++ b/tools/lib/find_bit.c

@@ -37,7 +37,12 @@ static unsigned long _find_next_bit(const unsigned long *addr,
 	if (!nbits || start >= nbits)
 		return nbits;

+#if (__BYTE_ORDER == __BIG_ENDIAN) && (BITS_PER_LONG != 64)
+	tmp = addr[(((nbits - 1)/BITS_PER_LONG) - (start / BITS_PER_LONG))]
+								^ invert;
+#else
 	tmp = addr[start / BITS_PER_LONG] ^ invert;
+#endif

 	/* Handle 1st word. */
 	tmp &= BITMAP_FIRST_WORD_MASK(start);

@@ -48,7 +53,12 @@ static unsigned long _find_next_bit(const unsigned long *addr,
 		if (start >= nbits)
 			return nbits;

+#if (__BYTE_ORDER == __BIG_ENDIAN) && (BITS_PER_LONG != 64)
+		tmp = addr[(((nbits - 1)/BITS_PER_LONG) - (start / BITS_PER_LONG))]
+								^ invert;
+#else
 		tmp = addr[start / BITS_PER_LONG] ^ invert;
+#endif
 	}

 	return min(start + __ffs(tmp), nbits);

@@ -75,8 +85,15 @@ unsigned long find_first_bit(const unsigned long *addr, unsigned long size)
 	unsigned long idx;

 	for (idx = 0; idx * BITS_PER_LONG < size; idx++) {
+#if (__BYTE_ORDER == __BIG_ENDIAN) && (BITS_PER_LONG != 64)
+		if (addr[(((size-1)/BITS_PER_LONG) - idx)])
+			return min(idx * BITS_PER_LONG +
+				__ffs(addr[(((size-1)/BITS_PER_LONG) - idx)]),
+									size);
+#else
 		if (addr[idx])
 			return min(idx * BITS_PER_LONG + __ffs(addr[idx]), size);
+#endif
 	}

 	return size;
--

1.9.1