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root@nixbox /opt/systemtap_scripts % stap -v -e 'probe kernel.function("vfs_read") {printf("%s\n", $$params); exit();}'
Pass 1: parsed user script and 76 library script(s) using 71020virt/21736res/2328shr kb, in 230usr/50sys/276real ms.
semantic error: unable to find local '$params' near pc 0xffffffff81115ac6  in  vfs_read fs/read_write.c ( (alternatives: $file $buf $count $pos $ret): identifier '$$params' at <input>:1:51
        source: probe kernel.function("vfs_read") {printf("%s\n", $$params); exit();}
                                                                  ^
Pass 2: analyzed script: 1 probe(s), 1 function(s), 0 embed(s), 0 global(s) using 261600virt/171100res/118832shr kb, in 900usr/220sys/1120real ms.
Pass 2: analysis failed.  Try again with another '--vp 01' option.