Valgrind (notite)

VALGRIND

- tools available: memcheck (for memory leaks), cachegrind (detects cache misses), massif (heap and stack profiling), helgrind, callgrind, DRD

- cannot be attached to a already running process (Valgrind wants to have full control from the very start)

- results are given for the whole process but you can find the errors coming from a specific shared object

Memcheck

> valgrind --tool=memcheck --leak-check=yes --track-children=yes --log-file=log.txt java Example

Types of errors (counting the occurrences):

• illegal read/write

    cat log.txt | grep "Invalid" | wc -l

• use of uninitialised values

    cat log.txt | grep "uninitialised" | wc -l 
Results at the end of log file:

• definitely lost = program is leaking memory
• indirectly = leaking memory in a pointer-based structure
• possibly = leaking memory unless pointers do something awesome
• still rechable = it just didn't free some memory it could have freed
• suppressed = to be ignored

Cachegrind

> valgrind --tool=cachegrind --log-file=log.txt java -Djava.library.path=/home/gtache/workspace/MKL/lib Example

Results in the log file, miss rates for L1, L2, L3. 

Massif

> valgrind --tool=massif --stacks=yes --time-unit=B --log-file=log.txt java --massif-out-file=massif.out.txt -Djava.library.path=/home/gtache/workspace/MKL/lib Example

Results in massif.out

> vim massif.out  or  ms_print massif.out # snapshots of total memory consumption for heap/stack at different moments of time

• mem_heap_B = the number of useful heap bytes allocated at that point
• mem_heap_extra_B = the number of bytes allocated in the excess of what the program asked for (might be associated with alignment)
• mem_stacks_B = size of the stack

Sometimes it gives the detailed heap tree for a snapshot.

> massif_visualizer massif.out # graphical output

Linux: send process to run in background

[ctr+z] to pause it
> bg # to send it in the background

Code jam, for women (solutiile mele, #1)

Problema 1) link

Enunt

Cody are mai multe produse in magazin si vrea sa faca o reducere de 75%. Ea tipareste niste etichete atat pt preturile vechi cat si pt cele noi, dar comanda ii vine cu preturile amestecate, de la cel mai mic la cel mai mare, iar ea trebuie sa gaseasca perechile (pret nou, pret vechi).

Idee

2 pointeri p1 si p2, unul aflat la inceput (pe pretul minim, care mereu va fi cel la reducere), iar altul flotant, in cautarea originalului de la pretul redus. Pointerii se misca pana cand ajung la celalalt capat.

Rezolvare (Python)

f = open("A-large-practice.in", 'r')
T = eval(f.readline())

for i in range (T):
N = eval(f.readline())
v = f.readline().split(' ')
p1 = 0
p2 = 1
print("Case #", i+1, ": ", sep="", end="")
while (p1 < 2*N):
nr = 4/3 * eval(v[p1]) # the previous price value
while (eval(v[p2]) < nr):
p2 += 1 # put p2 there
print(v[p1], " ", sep="", end="")
v[p2] = "-1" # mark as visited by p2
# reposition p1
p1 += 1
while (p1 < 2*N and eval(v[p1]) == -1):
p1 += 1 # skip where p2 already was positioned
print()

"Disable yama" in Ubuntu

 cat /proc/sys/kernel/yama/ptrace_scope 

As Linux grows in popularity, it will become a larger target for malware. One particularly troubling weakness of the Linux process interfaces is that a single user is able to examine the memory and running state of any of their processes. For example, if one application (e.g. Pidgin) was compromised, it would be possible for an attacker to attach to other running processes (e.g. Firefox, SSH sessions, GPG agent, etc) to extract additional credentials and continue to expand the scope of their attack without resorting to user-assisted phishing. This is not a theoretical problem. SSH session hijacking and arbitrary code injection attacks already exist and remain possible if ptrace is allowed to operate as before. Since ptrace is not commonly used by non-developers and non-admins, system builders should be allowed the option to disable this debugging system.

ptrace_scope value 0 => disable this security, while value 1 is the default after each booting

 echo 0 > /proc/sys/kernel/yama/ptrace_scope 

JVM