Re: [PATCH perf 3/4] perf tools: Fix unused variables: x86_{32,64}_regoffset_table

["Wangnan (F)" <wangnan0@huawei.com>]

On 2016/1/20 21:59, Arnaldo Carvalho de Melo wrote:
> Em Tue, Jan 19, 2016 at 09:33:06PM +0000, Ben Hutchings escreveu:
>> gcc 5 doesn't seem to care about these, but gcc 6 does and that
>> results in a build failure.
> Ben, please CC the people on the CC list for the patch that introduces
> the problem, Wang, He, can I have your Acked-by?
>
> - Arnaldo
>   

This patch lead me find a bug in original code.

If both perf and target ELF binary is x86_64, following command works okay:

  # perf probe -v -n --exec /tmp/oxygen_root/lib64/libc.so.6 pselect 
data exceptfds readfds writefds nfds sigmask tval timeout
  <SNIP>
  Opening /sys/kernel/debug/tracing//uprobe_events write=1
  Writing event: p:probe_libc/pselect 
/home/w00229757/oxygen_root-w00229757/lib64/libc-2.18.so:0xdfef0 
data=-216(%sp):u64 exceptfds=%cx:u64 readfds=%si:u64 writefds=%dx:u64 
nfds=%di:s32 sigmask=%r9:u64 tval=-232(%sp):u64 timeout=%r8:u64
  <SNIP>

But if the library is x86_32, result is incorrect:

   # perf probe -v -n --exec /tmp/oxygen_root/lib32/libc.so.6 pselect 
data exceptfds readfds writefds nfds sigmask tval
   <SNIP>
   Writing event: p:probe_libc/pselect 
/tmp/oxygen_root-w00229757/lib32/libc-2.18.so:0xd1330 data=-172(%si):u64 
exceptfds=+16(%si):u32 readfds=+8(%si):u32 writefds=+12(%si):u32 
nfds=+4(%si):s32 sigmask=+24(%si):u32 tval=-180(%si):u64 
timeout=+20(%si):u32
   <SNIP>

We know that (%si) is used to passing arguments. Here we should see 
'%sp' or '$stack'.

Use a x86_32 perf we get currect result:

  # ~/perf probe -v -n --exec /tmp/oxygen_root/lib32/libc.so.6 pselect 
data exceptfds readfds writefds nfds sigmask tval
  <SNIP>
  Writing event: p:probe_libc/pselect 
/tmp/oxygen_root-w00229757/lib32/libc-2.18.so:0xd1330 
data=-172($stack):u64 exceptfds=+16($stack):u32 readfds=+8($stack):u32 
writefds=+12($stack):u32 nfds=+4($stack):s32 sigmask=+24($stack):u32 
tval=-180($stack):u64
  <SNIP>


Use a small test program to check the result:

  #include <sys/time.h>
  #include <sys/types.h>
  #include <unistd.h>
  #include <memory.h>

  static struct {
         fd_set r, w, e;
         struct timespec ts;
         sigset_t m;
  } s;

  int main()
  {
         memset(&s, '\0', sizeof(s));

         pselect(0, &s.r, &s.w, &s.e, &s.ts, &s.m);
         return 0;
  }

# gcc -m32 -g ./test_pselect.c

Use x86_32 perf:

# ./perf probe -v  --exec /tmp/oxygen_root/lib32/libc.so.6 pselect data 
exceptfds readfds writefds nfds sigmask tval
Writing event: p:probe_libc/pselect 
/tmp/oxygen_root-w00229757/lib32/libc-2.18.so:0xd1330 
data=-172($stack):u64 exceptfds=+16($stack):u32 readfds=+8($stack):u32 
writefds=+12($stack):u32 nfds=+4($stack):s32 sigmask=+24($stack):u32 
tval=-180($stack):u64
Added new event:
   probe_libc:pselect   (on pselect in 
/tmp/oxygen_root-w00229757/lib32/libc-2.18.so with data exceptfds 
readfds writefds nfds sigmask tval)

You can now use it in all perf tools, such as:

     perf record -e probe_libc:pselect -aR sleep 1

# ./perf record -e probe_libc:pselect ./a.out
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.011 MB perf.data (1 samples) ]
# ./perf script
            a.out 25336 [006] 64588.457597: probe_libc:pselect: 
(f7663330) data=0xf772e00000000000 exceptfds=0x8049880 readfds=0x8049780 
writefds=0x8049800 nfds=0 sigmask=0x8049908 tval=0x0

Switch to x86_64 perf:

  # ./perf probe -v  --exec /tmp/oxygen_root/lib32/libc.so.6 pselect 
data exceptfds readfds writefds nfds sigmask tval
  <SNIP>
  Opening /sys/kernel/debug/tracing//uprobe_events write=1
Writing event: p:probe_libc/pselect 
/tmp/oxygen_root-w00229757/lib32/libc-2.18.so:0xd1330 data=-172(%si):u64 
exceptfds=+16(%si):u32 readfds=+8(%si):u32 writefds=+12(%si):u32 
nfds=+4(%si):s32 sigmask=+24(%si):u32 tval=-180(%si):u64
Added new event:
   probe_libc:pselect   (on pselect in 
/tmp/oxygen_root-w00229757/lib32/libc-2.18.so with data exceptfds 
readfds writefds nfds sigmask tval)

You can now use it in all perf tools, such as:

     perf record -e probe_libc:pselect -aR sleep 1

# ./perf record -e probe_libc:pselect ./a.out
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.011 MB perf.data (1 samples) ]
# ./perf script
            a.out 25599 [002] 64759.743554: probe_libc:pselect: 
(f76e7330) data=0x0 exceptfds=0x0 readfds=0x0 writefds=0x0 nfds=0 
sigmask=0x0 tval=0x0

Sad...

I think this problem is not introduced by my patch. In fact
there's a fundamental problem in get_arch_regstr() that it is
impossible to switch sub ISA.

Not only x86_64 and x86_32, I think on arm64 we also have this
problem when we try to setup uprobes on arm32 code. For me the
later problem is more important because there are many legacy arm32
applications on Android platform (and I have already seen the buggy
unwind result in this case. It is another problem though).

So I suggest us to solve this problem first before considering
gcc 6 Werror. At least x86_32_regoffset_table and x86_64_regoffset_table
should both be compiled no matter which ISA we select for perf.

Thank you.