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

["平松雅巳 / HIRAMATU，MASAMI" <masami.hiramatsu.pt@hitachi.com>]

>From: Wangnan (F) [mailto: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>

Ah, I see. Uprobes may not check the target binary is in 32bit mode.
Since the stack of x86-64 and x86-32 on pt_regs are different,
(regs->sp points stack on x86-64, &(regs->pt) points stack on x86-32)
uprobes would better checking and change the behavior.

But anyway, it is also fixed by changing perf's register table.

>
>
>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.

Right, but I guess this can fixed by switching %sp (for x86-64)
and +0(%sp) (for x86-32) instead of $stack.
 
Thanks!


>
>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.