Re: Cannot access data passed in via gettimeofday

[Eugene Teo <eteo at redhat dot com>]

Perry Cheng wrote:
> I am having trouble getting parameters out of what seems to be a simple 
> but buggy system-tap script.  The script is included below and the test 
> case (a short C program) follows.  Basically, I am trying to hi-jack 
> gettimeofday and piggyback some information back by treating the struct it 
> passes in as a larger buffer than struct timeval.  However, I need to 
> separate regular calls to gettimeofday from the special ones where the 
> special path triggers.  To do this, I though I could treat the struct as 
> an incoming parameter by looking for unusual bit patterns in the struct. 
> Unfortunately, I can't seem to see the data at all despite using the 
> copy_from_user function to copy data from user to kernel space.  Any idea 
> what's going on here?   I additionally hijack stime so i can distinguish 
> in the output my special call to gettimeofday.   The sample out below 
> shows that the special values 0xaaaaaaaa and 0xbbbbbbbb are not 
> transmitted.   If I use settimeofday instead of gettimeofday, then this 
> program seems to work.   It feels like there is some other mechanism at 
> work here that I don't know about. 

[eteo@kerndev tmp]$ stap -g test.stp -c ./test
Password:
---------------------------
gettimeofday 0:   sec = aaaaaaaa  usec = bbbbbbbb
---------------------------
gettimeofday 0:   sec = 23a  usec = 1
...

Eugene
-- 
1024D/58DF8823 print 47B9 90F6 AE4A 9C51 37E0  D6E1 EA84 C6A2 58DF 8823
main(i) { putchar(182623909 >> (i-1) * 5&31|!!(i<7)<<6) && main(++i); }