* Forw: [ahoole@islandnet.com: Accessing a va_list element from a probe]
@ 2014-01-05 20:18 Frank Ch. Eigler
2014-01-09 16:17 ` Frank Ch. Eigler
0 siblings, 1 reply; 2+ messages in thread
From: Frank Ch. Eigler @ 2014-01-05 20:18 UTC (permalink / raw)
To: systemtap
From: "Alexander M. Hoole" <ahoole@islandnet.com>
Subject: Accessing a va_list element from a probe
Date: Sun, 5 Jan 2014 11:50:39 -0800
[...]
I was wondering if you have ever attempted to access a va_list from a probe
in user-space. I can access the arguments; however, I'm presently puzzled
as to how to elegantly access the individual items of the va_list. For
example, if we setup a probe as follows:
.
probe process("/lib64/libc-2.17.so").function("printf")
{
printf("\tPID=%d, CMD=%s\n",pid(),execname())
printf("\tArgs: %s\n", $$parms)
printf("\tArg[1]: %s\n", user_string($format))
printf("\tArg[2]: %p\n", $arg)
}
.
According to stap -L we have the following arguments available:
.
# stap -L 'process("/lib64/libc-2.17.so").function("printf")'
process("/usr/lib64/libc-2.17.so").function("__printf@/usr/src/debug/glibc-2
.17-c758a686/stdio-common/printf.c:28") $format:char const* $arg:va_list
$done:int
.
If a program were to call something like the following:
.
printf("Person %s is %d years old\n", name, age);
.
Do you have any ideas how I could access name and age?
Any assistance would be appreciated.
Thanks,
Alex
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Forw: [ahoole@islandnet.com: Accessing a va_list element from a probe]
2014-01-05 20:18 Forw: [ahoole@islandnet.com: Accessing a va_list element from a probe] Frank Ch. Eigler
@ 2014-01-09 16:17 ` Frank Ch. Eigler
0 siblings, 0 replies; 2+ messages in thread
From: Frank Ch. Eigler @ 2014-01-09 16:17 UTC (permalink / raw)
To: systemtap, ahoole
Hi -
> I was wondering if you have ever attempted to access a va_list from a probe
> in user-space. I can access the arguments; however, I'm presently puzzled
> as to how to elegantly access the individual items of the va_list. [...]
>
> According to stap -L we have the following arguments available:
> # stap -L 'process("/lib64/libc-2.17.so").function("printf")'
> process("/usr/lib64/libc-2.17.so").function("__printf@/usr/src/debug/glibc-2
> .17-c758a686/stdio-common/printf.c:28") $format:char const* $arg:va_list
> $done:int
My guess is that one way to do this would be a tapset function kind of
like tokenize(), where some internal state is kept from call to call
(the va_list pointer), and consecutive varargs are fetched off of the
stack, based upon types & sizes, with user_long() etc. It'd have to
reimplement gcc __builtin_va_arg, which is not THAT bad, but a bit of
work.
- FChE
^ permalink raw reply [flat|nested] 2+ messages in thread
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