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Message-Id: <20160730120408.1165064035943d452bb377ee@kernel.org>
Date: Sat, 30 Jul 2016 12:04:08 +0900
From: Masami Hiramatsu <mhiramat@...nel.org>
To: Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, peterz@...radead.org,
mingo@...hat.com, acme@...nel.org,
alexander.shishkin@...ux.intel.com, mhiramat@...nel.org,
wangnan0@...wei.com, hemant@...ux.vnet.ibm.com
Subject: Re: [RFC] perf uprobe: Skip prologue if program compiled without
optimization
On Thu, 28 Jul 2016 20:01:51 +0530
Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com> 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@...ux.vnet.ibm.com>
> ---
> 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@...nel.org>
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