Re: [RFC] perf uprobe: Skip prologue if program compiled without optimization

[Ravi Bangoria]

On Saturday 30 July 2016 08:34 AM, Masami Hiramatsu wrote:

On Thu, 28 Jul 2016 20:01:51 +0530
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.

Right! :)


Thanks Masami for review.

I've sent patch with changes you suggested. Please review it.

-Ravi

Re: [RFC] perf uprobe: Skip prologue if program compiled without optimization

[Ravi Bangoria]

On Saturday 30 July 2016 08:34 AM, Masami Hiramatsu wrote:

On Thu, 28 Jul 2016 20:01:51 +0530
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.

Right! :)


Thanks Masami for review.

I've sent patch with changes you suggested. Please review it.

-Ravi

Re: [RFC] perf uprobe: Skip prologue if program compiled without optimization

[Masami Hiramatsu]

On Thu, 28 Jul 2016 20:01:51 +0530
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.

Right! :)

> For example,
>   $ vim test.c
> #include 
> 
> 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 e5mov%rsp,%rbp
>   40053a:   48 83 ec 10 sub-bashx10,%rsp
>   40053e:   89 7d fcmov%edi,-0x4(%rbp)
> /home/ravi/test.c:5
>   400541:   8b 45 fcmov-0x4(%rbp),%eax
> ...
> ...
> main():
> /home/ravi/test.c:9
>   400558:   55  push   %rbp
>   400559:   48 89 e5mov%rsp,%rbp
> /home/ravi/test.c:10
>   40055c:   bf 2a 00 00 00  mov-bashx2a,%edi
>   400561:   e8 d0 ff ff ff  callq  400536 
> /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:0x0536 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:0x0541 i=-4(%bp):s32
> 
>   $ ./perf record -e probe_test:foo ./test
>   $ ./perf script
> test  6300 [001]  5877.879327: probe_test:foo: (400541) i=42

It is great!
And I think we also should give a notice message for users about
skipping prologue, so that they can understand why the probe point
is not on the function entry address ;)


> 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 
> ---
>  tools/perf/util/probe-finder.c | 156 
> +
>  1 file changed, 156 insertions(+)
> 
> diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
> index f2d9ff0..a788b9c2 100644
> --- a/tools/perf/util/probe-finder.c
> +++ b/tools/perf/util/probe-finder.c
> @@ -892,6 +892,161 @@ static int find_probe_point_lazy(Dwarf_Die *sp_die, 
> struct probe_finder *pf)
>   return die_walk_lines(sp_die, probe_point_lazy_walker, pf);
>  }
>  
> +static bool var_has_loclist(Dwarf_Die *die)
> +{
> + Dwarf_Attribute loc;
> + int tag = dwarf_tag(die);
> +
> + if (tag != DW_TAG_formal_parameter &&
> + tag != DW_TAG_variable)
> + return false;
> +
> + return (dwarf_attr_integrate(die, DW_AT_location, ) &&
> + dwarf_whatform() == DW_FORM_sec_offset);
> +}
> +
> +/*
> + * For any object in given CU whose DW_AT_location is a location list,
> + * target program is compiled with optimization.
> + */

OK, anyway if it has loclist, we can ensure the variable is available at that 
address.

> +static bool optimized_target(Dwarf_Die *die)
> +{
> + Dwarf_Die tmp_die;
> +
> + if (var_has_loclist(die))
> + return true;
> +
> + if (!dwarf_child(die, _die) && optimized_target(_die))
> + return true;
> +
> + if (!dwarf_siblingof(die, _die) && optimized_target(_die))
> + return true;
> +
> + return false;
> +}
> +
> +static bool get_entrypc_idx(Dwarf_Lines *lines, unsigned long nr_lines,
> + Dwarf_Addr pf_addr, unsigned long *entrypc_idx)
> +{
> + unsigned long i;
> + Dwarf_Addr addr;
> +
> + for (i = 0; i < nr_lines; i++) {
> + if (dwarf_lineaddr(dwarf_onesrcline(lines, i), ))
> + return false;
> +
> + if (addr == pf_addr) {
> + *entrypc_idx = i;
> + return true;
> + }
> + }
> + return false;
> +}
> +
> +static bool get_postprologue_addr(unsigned long entrypc_idx,
> +   Dwarf_Lines *lines,
> +   unsigned long nr_lines,
> 

Re: [RFC] perf uprobe: Skip prologue if program compiled without optimization

[Masami Hiramatsu]

On Thu, 28 Jul 2016 20:01:51 +0530
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.

Right! :)

> For example,
>   $ vim test.c
> #include 
> 
> 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 e5mov%rsp,%rbp
>   40053a:   48 83 ec 10 sub-bashx10,%rsp
>   40053e:   89 7d fcmov%edi,-0x4(%rbp)
> /home/ravi/test.c:5
>   400541:   8b 45 fcmov-0x4(%rbp),%eax
> ...
> ...
> main():
> /home/ravi/test.c:9
>   400558:   55  push   %rbp
>   400559:   48 89 e5mov%rsp,%rbp
> /home/ravi/test.c:10
>   40055c:   bf 2a 00 00 00  mov-bashx2a,%edi
>   400561:   e8 d0 ff ff ff  callq  400536 
> /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:0x0536 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:0x0541 i=-4(%bp):s32
> 
>   $ ./perf record -e probe_test:foo ./test
>   $ ./perf script
> test  6300 [001]  5877.879327: probe_test:foo: (400541) i=42

It is great!
And I think we also should give a notice message for users about
skipping prologue, so that they can understand why the probe point
is not on the function entry address ;)


> 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 
> ---
>  tools/perf/util/probe-finder.c | 156 
> +
>  1 file changed, 156 insertions(+)
> 
> diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
> index f2d9ff0..a788b9c2 100644
> --- a/tools/perf/util/probe-finder.c
> +++ b/tools/perf/util/probe-finder.c
> @@ -892,6 +892,161 @@ static int find_probe_point_lazy(Dwarf_Die *sp_die, 
> struct probe_finder *pf)
>   return die_walk_lines(sp_die, probe_point_lazy_walker, pf);
>  }
>  
> +static bool var_has_loclist(Dwarf_Die *die)
> +{
> + Dwarf_Attribute loc;
> + int tag = dwarf_tag(die);
> +
> + if (tag != DW_TAG_formal_parameter &&
> + tag != DW_TAG_variable)
> + return false;
> +
> + return (dwarf_attr_integrate(die, DW_AT_location, ) &&
> + dwarf_whatform() == DW_FORM_sec_offset);
> +}
> +
> +/*
> + * For any object in given CU whose DW_AT_location is a location list,
> + * target program is compiled with optimization.
> + */

OK, anyway if it has loclist, we can ensure the variable is available at that 
address.

> +static bool optimized_target(Dwarf_Die *die)
> +{
> + Dwarf_Die tmp_die;
> +
> + if (var_has_loclist(die))
> + return true;
> +
> + if (!dwarf_child(die, _die) && optimized_target(_die))
> + return true;
> +
> + if (!dwarf_siblingof(die, _die) && optimized_target(_die))
> + return true;
> +
> + return false;
> +}
> +
> +static bool get_entrypc_idx(Dwarf_Lines *lines, unsigned long nr_lines,
> + Dwarf_Addr pf_addr, unsigned long *entrypc_idx)
> +{
> + unsigned long i;
> + Dwarf_Addr addr;
> +
> + for (i = 0; i < nr_lines; i++) {
> + if (dwarf_lineaddr(dwarf_onesrcline(lines, i), ))
> + return false;
> +
> + if (addr == pf_addr) {
> + *entrypc_idx = i;
> + return true;
> + }
> + }
> + return false;
> +}
> +
> +static bool get_postprologue_addr(unsigned long entrypc_idx,
> +   Dwarf_Lines *lines,
> +   unsigned long nr_lines,
> +   Dwarf_Addr highpc,
> +