fche changed the topic of #systemtap to: http://sourceware.org/systemtap; email systemtap@sourceware.org if answers here not timely, conversations may be logged
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<p53ud0ny1> How can i control the execution flow of some function? e.g. I want to intercept function foobar() and make it immediatly return value x.
<p53ud0ny1> I know how to probe the function, but how can i modify it's behavoir?
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<prerna> hi, I'm getting this error when I compile my systemtap script :
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<prerna> Makefile:619: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
<prerna> Makefile:619: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
<prerna> Makefile:619: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
<prerna> Makefile:619: Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong not supported by compiler
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<prerna> Cannot use CONFIG_CC_STACKPROTECTOR_STRONG : -fstack_protector_strong not supported
<prerna> (sorry for the multiple copy)
<prerna> Is there a way for me to turn it off ?
<prerna> btw, my script is just running userspace probe:
<prerna> stap -k -vve 'probe process("/usr/libexec/qemu-kvm").mark("scsi_req_parsed") {print("Hi");} '
<lorddoskias1> p53ud0ny1: apparently this is not possible :) at least in kernel mode ;0
<lorddoskias1> prerna: try upgrading your compiler?
<prerna> lorddoskias1, that isn't an option :) I was wondering if I could introduce a new flag that said "-Wnofstack-protector-strong" or somesuch
<lorddoskias1> funny
<lorddoskias1> i can't find any reference to fstack-protector-strong in current systemtap source
<prerna> exactly
<prerna> so I am unable to understand how this flag got picked
<lorddoskias1> maybe the process it's probing is compiled with this flag? e.g. qemu-vkm?
<lorddoskias1> objdump -s --section .comment /path/binary
<lorddoskias1> what does this show?
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<lorddoskias1> right, this just shows the gcc version
<lorddoskias1> so CONFIG_CC_STACKPROTECTOR_STRONG comes from your kernel config
<prerna> hmm, but this is a userspace probe script
<prerna> that was another thing -- systematic refused to process my script until I installed kernel-headers/devel/debuginfo.. and the script in question needed none of that
<prerna> or did it ?
<prerna> I mean, do userspace probes have any dependence on kernel-devel ?
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<lorddoskias1> ops, got too eager with ctrl+w ;D
<lorddoskias1> so yes, uprobes are a kernel facility: https://lwn.net/Articles/499190/
<lorddoskias1> as such in order to probe a userspace process you do have to compile a kernel module
<prerna> hmm, that explains
<lorddoskias1> i have never dealt with uspace probing
<prerna> so I installed another gcc under /opt/rh/devtoolset-3/ , modified PATH so that this was picked , but systemtap keeps giving me the same error
<lorddoskias1> i had the same problem :)
<prerna> lorddoskias1, thanks, but you've been very helpful :)
<lorddoskias1> so what you have to do is
<lorddoskias1> stap -BCC=/path/to/dev/tool/set
<lorddoskias1> or rather /path/to/devtoolset/gcc
<prerna> wow thanks,trying
<prerna> I got another unrelated error, but naps, this helps !
<prerna> thanks a tonne'
<prerna> s/naps/nps
<lorddoskias1> i only knew this, since i had to also use the devtoolset rathen than the default compiler
<p53ud0ny1> lorddoskias1: Thanks. How about just modifying data at the kernel level? e.g. Changing a functions paramaters at a particular line.
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<lorddoskias1> p53ud0ny1: you could potentially do that, depending on the visibility of the data in the debug symbols
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<fche> p53ud0ny1, # stap -g -e 'probe kernel.function("foo") { $param = 5 }' is the general pattern
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