On Fri, 6 Jan 2023 11:52:19 +1300 Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@xxxxxxxxx> wrote: > In tracecmd/trace-record.c:<run_cmd>, trace-cmd record -F <executable> > is launched via the libc's execvp() routine. > > If you run a command which is meant to be found in one of the entries > of the PATH env var then you should see <n> invocations of the > __x64_sys_execve() syscall where <n> will be equal to the number of > attempts that execvp() routine had done until it could find the > executable. > > While the routine works as expected, its behaviour can seem a bit > cryptic for untrained eyes and makes one wonder whether there is > something wrong in any of the parts involved in the tracing operation. > Some time after one digs into trace-cmd's and libc's code base, one > realises that everything is working as expected but documenting it could > save other people's time :-) > > Document the expected behaviour ftrace users should see depending on > the way -F <executable> is used. > > Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@xxxxxxxxx> > --- > Additional context: > > - absolute path example: > > # trace-cmd record -p function_graph \ > -g __x64_sys_execve -O nofuncgraph-irqs \ > -n __cond_resched --max-graph-depth 1 \ > -F /usr/bin/echo "ftrace" > /dev/null > > # trace-cmd report > echo-172994 [000] 185539.798539: funcgraph_entry: ! 803.376 us | __x64_sys_execve(); > > - PATH-dependent path example: > > # trace-cmd record -p function_graph \ > -g __x64_sys_execve -O nofuncgraph-irqs \ > -n __cond_resched --max-graph-depth 1 \ > -F echo "ftrace" > /dev/null > > # trace-cmd report > echo-172656 [002] 185009.671586: funcgraph_entry: ! 288.732 us | __x64_sys_execve(); > echo-172656 [002] 185009.671879: funcgraph_entry: ! 158.337 us | __x64_sys_execve(); > echo-172656 [002] 185009.672042: funcgraph_entry: ! 161.843 us | __x64_sys_execve(); > echo-172656 [002] 185009.672207: funcgraph_entry: ! 157.656 us | __x64_sys_execve(); > echo-172656 [002] 185009.672369: funcgraph_entry: ! 156.343 us | __x64_sys_execve(); > echo-172656 [002] 185009.672529: funcgraph_entry: ! 863.629 us | __x64_sys_execve(); Honestly, the above should be in the change log. > > --- > Documentation/trace-cmd/trace-cmd-record.1.txt | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/Documentation/trace-cmd/trace-cmd-record.1.txt b/Documentation/trace-cmd/trace-cmd-record.1.txt > index b709e48..a000cbe 100644 > --- a/Documentation/trace-cmd/trace-cmd-record.1.txt > +++ b/Documentation/trace-cmd/trace-cmd-record.1.txt > @@ -113,6 +113,10 @@ OPTIONS > Using *-F* will let you trace only events that are caused by the given > command. > > + Note: if the specified filename is neither absolute or relative then libc > + will invoke execve() syscall for every entry in the colon-separated list of > + directory pathnames specified in the PATH environment variable. > + Hmm, do you think it may be worth open-coding execvp() and looking for it from trace-cmd, and then only enabling tracing when it found the full path? -- Steve > *-P* 'pid':: > Similar to *-F* but lets you specify a process ID to trace. >
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