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

[Masami Hiramatsu]

On Thu, 28 Jul 2016 20:01:51 +0530
Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxxxxxxx> 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 <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

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 <ravi.bangoria@xxxxxxxxxxxxxxxxxx>
> ---
>  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, &loc) &&
> +		dwarf_whatform(&loc) == 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, &tmp_die) && optimized_target(&tmp_die))
> +		return true;
> +
> +	if (!dwarf_siblingof(die, &tmp_die) && optimized_target(&tmp_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), &addr))
> +			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,
> +				  Dwarf_Addr *postprologue_addr)
> +{
> +	unsigned long i;
> +	int entrypc_lno, lno;
> +	Dwarf_Line *line;
> +	Dwarf_Addr addr;
> +	bool p_end;
> +
> +	/* entrypc_lno is actual source line number */
> +	line = dwarf_onesrcline(lines, entrypc_idx);
> +	if (dwarf_lineno(line, &entrypc_lno))
> +		return false;
> +
> +	for (i = entrypc_idx; i < nr_lines; i++) {
> +		line = dwarf_onesrcline(lines, i);
> +
> +		if (dwarf_lineaddr(line, &addr) ||
> +		    dwarf_lineno(line, &lno)    ||
> +		    dwarf_lineprologueend(line, &p_end))
> +			return false;
> +
> +		/* highpc is exclusive. [entrypc,highpc) */
> +		if (addr >= highpc)
> +			break;
> +
> +		/* clang supports prologue-end marker */
> +		if (p_end)
> +			break;
> +
> +		/* Actual next line in source */
> +		if (lno != entrypc_lno)
> +			break;
> +
> +		/*
> +		 * Single source line can have multiple line records.
> +		 * For Example,
> +		 *     void foo() { printf("hello\n"); }
> +		 * contains two line records. One points to declaration and
> +		 * other points to printf() line. Variable 'lno' won't get
> +		 * incremented in this case but 'i' will.
> +		 */
> +		if (i != entrypc_idx)
> +			break;
> +	}
> +
> +	dwarf_lineaddr(line, postprologue_addr);
> +	if (*postprologue_addr >= highpc)
> +		dwarf_lineaddr(dwarf_onesrcline(lines, i - 1),
> +			       postprologue_addr);
> +
> +	return true;
> +}
> +
> +static void __skip_prologue(Dwarf_Die *sp_die, struct probe_finder *pf)
> +{
> +	unsigned long nr_lines = 0, entrypc_idx = 0;
> +	Dwarf_Lines *lines = NULL;
> +	Dwarf_Addr postprologue_addr;
> +	Dwarf_Addr highpc;
> +
> +	if (dwarf_highpc(sp_die, &highpc))
> +		return;
> +
> +	if (dwarf_getsrclines(&pf->cu_die, &lines, &nr_lines))
> +		return;
> +
> +	if (!get_entrypc_idx(lines, nr_lines, pf->addr, &entrypc_idx))
> +		return;
> +
> +	if (!get_postprologue_addr(entrypc_idx, lines, nr_lines,
> +				   highpc, &postprologue_addr))
> +		return;
> +
> +	pf->addr = postprologue_addr;
> +}
> +
> +static void skip_prologue(Dwarf_Die *sp_die, struct probe_finder *pf)
> +{
> +	struct perf_probe_point *pp = &pf->pev->point;
> +
> +	/* Not uprobe? */
> +	if (!pf->pev->uprobes)
> +		return;
> +
> +	/* Compiled with optimization? */
> +	if (optimized_target(&pf->cu_die))
> +		return;
> +
> +	/* Don't know entrypc? */
> +	if (!pf->addr)
> +		return;
> +
> +	/* Only FUNC and FUNC@SRC are eligible. */
> +	if (!pp->function || pp->line || pp->retprobe || pp->lazy_line ||
> +	    pp->offset || pp->abs_address)
> +		return;
> +
> +	__skip_prologue(sp_die, pf);
> +}
> +
>  static int probe_point_inline_cb(Dwarf_Die *in_die, void *data)
>  {
>  	struct probe_finder *pf = data;
> @@ -954,6 +1109,7 @@ static int probe_point_search_cb(Dwarf_Die *sp_die, void *data)
>  		if (pp->lazy_line)
>  			param->retval = find_probe_point_lazy(sp_die, pf);
>  		else {
> +			skip_prologue(sp_die, pf);

Hmm, this seems no good. prologue skipping must be done only if the function
parameter is specified. Please consider the case of probing prologue like single
stepping debug.

Thank you,


>  			pf->addr += pp->offset;
>  			/* TODO: Check the address in this function */
>  			param->retval = call_probe_finder(sp_die, pf);
> -- 
> 2.5.5
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>