Re: [PATCH] perf probe powerpc: Fix trace event post-processing

[Naveen N. Rao]

Sandipan Das wrote:
> In some cases, a symbol may have multiple aliases. Attempting to add
> an entry probe for such symbols results in a probe being added at an
> incorrect location while it fails altogether for return probes. This
> is only applicable for binaries with debug information.
>
> During the arch-dependent post-processing, the offset from the start
> of the symbol at which the probe is to be attached is determined and
> added to the start address of the symbol to get the probe's location.
> In case there are multiple aliases, this offset gets added multiple
> times for each alias of the symbol and we end up with an incorrect
> probe location.
>
> This can be verified on a powerpc64le system as shown below.
>
>   $ nm /lib/modules/$(uname -r)/build/vmlinux | grep "sys_open$"
>   ...
>   c000000000414290 T __se_sys_open
>   c000000000414290 T sys_open
>
>   $ objdump -d /lib/modules/$(uname -r)/build/vmlinux | grep -A 10 "<__se_sys_open>:"
>
>   c000000000414290 <__se_sys_open>:
>   c000000000414290:       19 01 4c 3c     addis   r2,r12,281
>   c000000000414294:       70 c4 42 38     addi    r2,r2,-15248
>   c000000000414298:       a6 02 08 7c     mflr    r0
>   c00000000041429c:       e8 ff a1 fb     std     r29,-24(r1)
>   c0000000004142a0:       f0 ff c1 fb     std     r30,-16(r1)
>   c0000000004142a4:       f8 ff e1 fb     std     r31,-8(r1)
>   c0000000004142a8:       10 00 01 f8     std     r0,16(r1)
>   c0000000004142ac:       c1 ff 21 f8     stdu    r1,-64(r1)
>   c0000000004142b0:       78 23 9f 7c     mr      r31,r4
>   c0000000004142b4:       78 1b 7e 7c     mr      r30,r3
>
>   For both the entry probe and the return probe, the probe location
>   should be _text+4276888 (0xc000000000414298). Since another alias
>   exists for 'sys_open', the post-processing code will end up adding
>   the offset (8 for powerpc64le) twice and perf will attempt to add
>   the probe at _text+4276896 (0xc0000000004142a0) instead.
>
> Before:
>
>   # perf probe -v -a sys_open
>
>   probe-definition(0): sys_open
>   symbol:sys_open file:(null) line:0 offset:0 return:0 lazy:(null)
>   0 arguments
>   Looking at the vmlinux_path (8 entries long)
>   Using /lib/modules/4.18.0-rc8+/build/vmlinux for symbols
>   Open Debuginfo file: /lib/modules/4.18.0-rc8+/build/vmlinux
>   Try to find probe point from debuginfo.
>   Symbol sys_open address found : c000000000414290
>   Matched function: __se_sys_open [2ad03a0]
>   Probe point found: __se_sys_open+0
>   Found 1 probe_trace_events.
>   Opening /sys/kernel/debug/tracing/kprobe_events write=1
>   Writing event: p:probe/sys_open _text+4276896
>   Added new event:
>     probe:sys_open       (on sys_open)
>   ...
>
>   # perf probe -v -a sys_open%return $retval
>
>   probe-definition(0): sys_open%return
>   symbol:sys_open file:(null) line:0 offset:0 return:1 lazy:(null)
>   0 arguments
>   Looking at the vmlinux_path (8 entries long)
>   Using /lib/modules/4.18.0-rc8+/build/vmlinux for symbols
>   Open Debuginfo file: /lib/modules/4.18.0-rc8+/build/vmlinux
>   Try to find probe point from debuginfo.
>   Symbol sys_open address found : c000000000414290
>   Matched function: __se_sys_open [2ad03a0]
>   Probe point found: __se_sys_open+0
>   Found 1 probe_trace_events.
>   Opening /sys/kernel/debug/tracing/README write=0
>   Opening /sys/kernel/debug/tracing/kprobe_events write=1
>   Parsing probe_events: p:probe/sys_open _text+4276896
>   Group:probe Event:sys_open probe:p
>   Writing event: r:probe/sys_open__return _text+4276896
>   Failed to write event: Invalid argument
>     Error: Failed to add events. Reason: Invalid argument (Code: -22)
>
> After:
>
>   # perf probe -v -a sys_open
>
>   probe-definition(0): sys_open
>   symbol:sys_open file:(null) line:0 offset:0 return:0 lazy:(null)
>   0 arguments
>   Looking at the vmlinux_path (8 entries long)
>   Using /lib/modules/4.18.0-rc8+/build/vmlinux for symbols
>   Open Debuginfo file: /lib/modules/4.18.0-rc8+/build/vmlinux
>   Try to find probe point from debuginfo.
>   Symbol sys_open address found : c000000000414290
>   Matched function: __se_sys_open [2ad03a0]
>   Probe point found: __se_sys_open+0
>   Found 1 probe_trace_events.
>   Opening /sys/kernel/debug/tracing/kprobe_events write=1
>   Writing event: p:probe/sys_open _text+4276888
>   Added new event:
>     probe:sys_open       (on sys_open)
>   ...
>
>   # perf probe -v -a sys_open%return $retval
>
>   probe-definition(0): sys_open%return
>   symbol:sys_open file:(null) line:0 offset:0 return:1 lazy:(null)
>   0 arguments
>   Looking at the vmlinux_path (8 entries long)
>   Using /lib/modules/4.18.0-rc8+/build/vmlinux for symbols
>   Open Debuginfo file: /lib/modules/4.18.0-rc8+/build/vmlinux
>   Try to find probe point from debuginfo.
>   Symbol sys_open address found : c000000000414290
>   Matched function: __se_sys_open [2ad03a0]
>   Probe point found: __se_sys_open+0
>   Found 1 probe_trace_events.
>   Opening /sys/kernel/debug/tracing/README write=0
>   Opening /sys/kernel/debug/tracing/kprobe_events write=1
>   Parsing probe_events: p:probe/sys_open _text+4276888
>   Group:probe Event:sys_open probe:p
>   Writing event: r:probe/sys_open__return _text+4276888
>   Added new event:
>     probe:sys_open__return (on sys_open%return)
>   ...
>
> Fixes: 99e608b5954c ("perf probe ppc64le: Fix probe location when using DWARF")
> Reported-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Sandipan Das <sandipan@xxxxxxxxxxxxx>

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

- Naveen