Re: [PATCH] perf test: Test case 66 broken on s390 (lib/traceevent issue)

Steven Rostedt
2020-01-24T15:07:46Z

On Fri, 24 Jan 2020 08:39:41 +0100
Thomas Richter <tmricht@linux.ibm.com> wrote:

> Test case 66 'Use vfs_getname probe to get syscall args filenames'
> is broken on s390, but works on x86. The test case fails with:
>
> [root@m35lp76 perf]# perf test -F 66
> 66: Use vfs_getname probe to get syscall args filenames
> :Recording open file:
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.004 MB /tmp/__perf_test.perf.data.TCdYj\
> (20 samples) ]
> Looking at perf.data file for vfs_getname records for the file we touched:
> FAILED!
> [root@m35lp76 perf]#
>
> The root cause of this failure has to do with
> commit 88903c464321c ("tracing/probe: Add ustring type for user-space string")
> and these changes:
> + if (strcmp(parg->type->name, "string") == 0)
> + fmt = ", __get_str(%s)";
> + else
> + fmt = ", REC->%s";
>

You confused me because those changes are not in commit 88903c464321c.
They are in 40b53b771806b ("tracing: probeevent: Add array type
support").

But reading what you wrote a second time, I'm assuming you mean the
user-space string changes and the above changes together make the
problem.

> On x86 __get_str() function is used to print the file name. This is ok
> for x86 because kernel and user space are in the same adddress space,
> the high bit of the address determines kernel vs user space addresses.
>
> This approach does not work on s390. On s390 kernel und user space are
> in different address spaces, both start with address 0x0. Which address
> space is currently active is stored in the processor status word (PSW).
>
> Therefore s390 must use 'ustring' for lookup of filename in the user
> space address space.
>
> Setting up the kprobe event using perf command:
>
> # ./perf probe "vfs_getname=getname_flags:72 pathname=filename:ustring"
>
> generates this format file:
> [root@m35lp76 perf]# cat /sys/kernel/debug/tracing/events/probe/\
> vfs_getname/format
> name: vfs_getname
> ID: 1172
> format:
> field:unsigned short common_type; offset:0; size:2; signed:0;
> field:unsigned char common_flags; offset:2; size:1; signed:0;
> field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
> field:int common_pid; offset:4; size:4; signed:1;
>
> field:unsigned long __probe_ip; offset:8; size:8; signed:0;
> field:__data_loc char[] pathname; offset:16; size:4; signed:1;
>
> print fmt: "(%lx) pathname=\"%s\"", REC->__probe_ip, REC->pathname

Hmm, that format looks wrong.

> [root@m35lp76 perf]#
>
> The difference between 'ustring' and 'string' is the print fmt statement
> in the last line. Using 'string' generates
>
> print fmt: "(%lx) pathname=\"%s\"", REC->__probe_ip, __get_str(pathname)
>
> This does not work on s390 because __get_str() function does not
> know the address space currently being used.

I don't think this is the issue, as __get_str() is just a way to handle
dynamic sized strings in the ring buffer. It shouldn't care about
kernel vs user space addresses at all.

>
> This is the command to generate the perf.data file:
>
> # ./perf record -e probe:vfs_getname -- touch /tmp/xxx
>
> This command creates the perf.data file which contains the contents
> for the kprobe event format file. The call chain is:
>
> main
> ....
> perf_session__open
> perf_header__process_sections (feature 1 tracing)
> perf_file_section__process
> trace_report
> read_event_files
> read_event_file
> parse_event_file
> tep_parse_event --> now in lib/traceevent/event-parse.c
> __parse_event
> tep_parse_format
> event_read_format
> event_read_print
> event_read_print_args
> process_args
> process_arg_token --> processes REC->xxx entry of event format file
> 'print fmt' line
> process_entry --> generates argument of type TEP_PRINT_FIELD entry
> with TEP_FIELD_IS_STRING set in arg->field.field->flags
> member
>
> Now the difference in the format file between
> [x86] print fmt: "(%lx) pathname=\"%s\"", ..., __get_str(pathname)
> and
> [s390] print fmt: "(%lx) pathname=\"%s\"", ..., REC->pathname
> comes into play.
>
> In the x86 case the function process_arg_token()
> handles in the case TEP_EVENT_ITEM the token "__get_str" and calls
> functions
>
> process_function()
> +--> process_str()
>
> and creates a print argument structure struct tep_print_arg with
> type TEP_PRINT_STRING.
> This is handled correctly when perf report later prints the argument using
> function print_str_arg() in file lib/traceevent/event-parse.c
> I omit the 35+ plus function call stack of the gdb where command.
>
> In the s390 case the function process_arg_token()
> handles in the case TEP_EVENT_ITEM the token "REC" and calls
> function
> process_entry()
>
> This creates a print argument structure struct tep_print_arg with
> type TEP_PRINT_FIELD and bit TEP_FIELD_IS_STRING set in flags.
>
> This is handled ***incorrectly** when perf report later prints the
> argument using function print_str_arg() in file
> lib/traceevent/event-parse.c. There is no support to print a string
> when type is TEP_PRINT_FIELD and bit TEP_FIELD_IS_STRING inflags is set.
> Again I omit the 35+ plus function call stack of the gdb where command.
>
> Output before:
> [root@m35lp76 perf]# perf report --stdio | egrep '^(# Samp| +[1-9.])+'
> # Samples: 20 of event 'probe:vfs_getname'
> 100.00% (4d2c32) pathname=""
>
> Output after:
> [root@m35lp76 perf]# ./perf report --stdio | egrep '^(# Samp| +[1-9.])+'
> # Samples: 20 of event 'probe:vfs_getname'
> 5.00% (4d2c32) pathname="/etc/ld.so.cache"
> 5.00% (4d2c32) pathname="/etc/ld.so.preload"
> 5.00% (4d2c32) pathname="/lib64/libc.so.6"
> 5.00% (4d2c32) pathname="/tmp/xxx"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_ADDRESS"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_COLLATE"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_CTYPE"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_IDENTIFICATION"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_MEASUREMENT"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_MESSAGES"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_MESSAGES/SYS_LC_MESSAGES"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_MONETARY"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_NAME"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_NUMERIC"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_PAPER"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_TELEPHONE"
> 5.00% (4d2c32) pathname="/usr/lib/locale/en_US.utf8/LC_TIME"
> 5.00% (4d2c32) pathname="/usr/lib/locale/locale-archive"
> 5.00% (4d2c32) pathname="/usr/lib64/gconv/gconv-modules.cache"
> 5.00% (4d2c32) pathname="/usr/share/locale/locale.alias"
> [root@m35lp76 perf]#
>
> Fix this by adding print string support for TEP_FIELD_IS_STRING
> bit set in flags and case TEP_PRINT_FIELD.
>
> Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
> ---
> tools/lib/traceevent/event-parse.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/tools/lib/traceevent/event-parse.c b/tools/lib/traceevent/event-parse.c
> index beaa8b8c08ff..68334f860b27 100644
> --- a/tools/lib/traceevent/event-parse.c
> +++ b/tools/lib/traceevent/event-parse.c
> @@ -4003,6 +4003,15 @@ static void print_str_arg(struct trace_seq *s, void *data, int size,
> trace_seq_printf(s, "%llx", addr);
> break;
> }
> + if (field->flags & TEP_FIELD_IS_STRING) {
> + int str_offset;
> +
> + str_offset = tep_data2host4(tep, *(unsigned int *)(data + arg->field.field->offset));
> + str_offset &= 0xffff;
> + print_str_to_seq(s, format, len_arg,
> + ((char *)data) + str_offset);
> + break;

This will break static strings.

This looks like a kernel bug, not a libtraceevent parsing bug.

> + }
> str = malloc(len + 1);
> if (!str) {
> do_warning_event(event, "%s: not enough memory!",

Does this patch fix it for you?

-- Steve

diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
index 905b10af5d5c..d3309fceb480 100644
--- a/kernel/trace/trace_probe.c
+++ b/kernel/trace/trace_probe.c
@@ -876,7 +876,8 @@ static int __set_print_fmt(struct trace_probe *tp, char *buf, int len,
for (i = 0; i < tp->nr_args; i++) {
parg = tp->args + i;
if (parg->count) {
- if (strcmp(parg->type->name, "string") == 0)
+ if ((strcmp(parg->type->name, "string") == 0) ||
+ (strcmp(parg->type->name, "ustring") == 0))
fmt = ", __get_str(%s[%d])";
else
fmt = ", REC->%s[%d]";
@@ -884,7 +885,8 @@ static int __set_print_fmt(struct trace_probe *tp, char *buf, int len,
pos += snprintf(buf + pos, LEN_OR_ZERO,
fmt, parg->name, j);
} else {
- if (strcmp(parg->type->name, "string") == 0)
+ if ((strcmp(parg->type->name, "string") == 0) ||
+ (strcmp(parg->type->name, "ustring") == 0))
fmt = ", __get_str(%s)";
else
fmt = ", REC->%s";