Re: [PATCH v2 2/2] perf uprobe: Skip prologue if program compiled without optimization

[Naveen N. Rao]

On 2016/08/03 02:28PM, Ravi Bangoria wrote:
> Function prologue prepares stack and registers before executing function
> logic. When target program is compiled without optimization, function
> parameter information is only valid after prologue. When we probe entrypc
> of the function, and try to record function parameter, it contains
> garbage value.
> 
> For example,
>   $ vim test.c
>     #include <stdio.h>
> 
>     void foo(int i)
>     {
>        printf("i: %d\n", i);
>     }
> 
>     int main()
>     {
>       foo(42);
>       return 0;
>     }
> 
>   $ gcc -g test.c -o test
>   $ objdump -dl test | less
>     foo():
>     /home/ravi/test.c:4
>       400536:       55                      push   %rbp
>       400537:       48 89 e5                mov    %rsp,%rbp
>       40053a:       48 83 ec 10             sub    -bashx10,%rsp
>       40053e:       89 7d fc                mov    %edi,-0x4(%rbp)
>     /home/ravi/test.c:5
>       400541:       8b 45 fc                mov    -0x4(%rbp),%eax
>     ...
>     ...
>     main():
>     /home/ravi/test.c:9
>       400558:       55                      push   %rbp
>       400559:       48 89 e5                mov    %rsp,%rbp
>     /home/ravi/test.c:10
>       40055c:       bf 2a 00 00 00          mov    -bashx2a,%edi
>       400561:       e8 d0 ff ff ff          callq  400536 <foo>
>     /home/ravi/test.c:11
> 
>   $ ./perf probe -x ./test 'foo i'
>   $ cat /sys/kernel/debug/tracing/uprobe_events
>      p:probe_test/foo /home/ravi/test:0x0000000000000536 i=-12(%sp):s32
> 
>   $ ./perf record -e probe_test:foo ./test
>   $ ./perf script
>      test  5778 [001]  4918.562027: probe_test:foo: (400536) i=0
> 
> Here variable 'i' is passed via stack which is pushed on stack at
> 0x40053e. But we are probing at 0x400536.
> 
> To resolve this issues, we need to probe on next instruction after
> prologue. gdb and systemtap also does same thing. I've implemented
> this patch based on approach systemtap has used.
> 
> After applying patch:
> 
>   $ ./perf probe -x ./test 'foo i'
>   $ cat /sys/kernel/debug/tracing/uprobe_events
>     p:probe_test/foo /home/ravi/test:0x0000000000000541 i=-4(%bp):s32
> 
>   $ ./perf record -e probe_test:foo ./test
>   $ ./perf script
>     test  6300 [001]  5877.879327: probe_test:foo: (400541) i=42
> 
> No need to skip prologue for optimized case since debug info is correct
> for each instructions for -O2 -g. For more details please visit:
>         https://bugzilla.redhat.com/show_bug.cgi?id=612253#c6
> 
> Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxxxxxxx>
> ---
> Changes in v2:
>   - Skipping prologue only when any ARG is either C variable, $params
>     or $vars.
>   - Probe on line(:1) may not be always possible. Recommend only address
>     to force probe on function entry.
> 
>  tools/perf/util/probe-finder.c | 164 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 164 insertions(+)

This fixes the uprobe issue reported by Michael Petlan:
https://www.mail-archive.com/linux-perf-users@xxxxxxxxxxxxxxx/msg02348.html

Acked-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>