No system call when malloc after free
While I was playing a little with kernel audit system, I made a small C program:
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char** argv){
void *t;
while(1){
t = malloc(1);
free(t);
}
return 0;
}
And applied the following filters to audit:
-a always,exit -F arch=b32 -S open,openat -F exit=-EACCES -F key=access
-a always,exit -F arch=b64 -S open,openat -F exit=-EACCES -F key=access
-a always,exit -F arch=b32 -S brk
-a always,exit -F arch=b64 -S brk
After compiling and running, I noticed that sys_brk wasn't showing up in the audit log.
Furthermore it didn't also appear in strace, even tho malloc was called (checked with ltrace).
Lastly I removed the free and the calls to sys_brk started showing up.
What is causing this type of behaviour? Does glibc make some kind of optimization in malloc and free functions to prevent useless syscalls?
TL;DR: free followed by malloc makes neither call the kernel. Why?
linux linux-kernel linux-audit
edited 56 mins ago
Rui F Ribeiro
41.2k1481139
asked 1 hour ago
Jorge HelenoJorge Heleno
1114
add a comment |
While I was playing a little with kernel audit system, I made a small C program:
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char** argv){
void *t;
while(1){
t = malloc(1);
free(t);
}
return 0;
}
And applied the following filters to audit:
-a always,exit -F arch=b32 -S open,openat -F exit=-EACCES -F key=access
-a always,exit -F arch=b64 -S open,openat -F exit=-EACCES -F key=access
-a always,exit -F arch=b32 -S brk
-a always,exit -F arch=b64 -S brk
After compiling and running, I noticed that sys_brk wasn't showing up in the audit log.
Furthermore it didn't also appear in strace, even tho malloc was called (checked with ltrace).
Lastly I removed the free and the calls to sys_brk started showing up.
What is causing this type of behaviour? Does glibc make some kind of optimization in malloc and free functions to prevent useless syscalls?
TL;DR: free followed by malloc makes neither call the kernel. Why?
linux linux-kernel linux-audit
edited 56 mins ago
Rui F Ribeiro
41.2k1481139
asked 1 hour ago
Jorge HelenoJorge Heleno
1114
It might use brk () or mmap ()
– 炸鱼薯条德里克
1 hour ago
Neither appeared in strace
– Jorge Heleno
15 mins ago
add a comment |
While I was playing a little with kernel audit system, I made a small C program:
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char** argv){
void *t;
while(1){
t = malloc(1);
free(t);
}
return 0;
}
And applied the following filters to audit:
-a always,exit -F arch=b32 -S open,openat -F exit=-EACCES -F key=access
-a always,exit -F arch=b64 -S open,openat -F exit=-EACCES -F key=access
-a always,exit -F arch=b32 -S brk
-a always,exit -F arch=b64 -S brk
After compiling and running, I noticed that sys_brk wasn't showing up in the audit log.
Furthermore it didn't also appear in strace, even tho malloc was called (checked with ltrace).
Lastly I removed the free and the calls to sys_brk started showing up.
What is causing this type of behaviour? Does glibc make some kind of optimization in malloc and free functions to prevent useless syscalls?
TL;DR: free followed by malloc makes neither call the kernel. Why?
linux linux-kernel linux-audit
edited 56 mins ago
Rui F Ribeiro
41.2k1481139
asked 1 hour ago
Jorge HelenoJorge Heleno
1114
While I was playing a little with kernel audit system, I made a small C program:
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char** argv){
void *t;
while(1){
t = malloc(1);
free(t);
}
return 0;
}
And applied the following filters to audit:
-a always,exit -F arch=b32 -S open,openat -F exit=-EACCES -F key=access
-a always,exit -F arch=b64 -S open,openat -F exit=-EACCES -F key=access
-a always,exit -F arch=b32 -S brk
-a always,exit -F arch=b64 -S brk
After compiling and running, I noticed that sys_brk wasn't showing up in the audit log.
Furthermore it didn't also appear in strace, even tho malloc was called (checked with ltrace).
Lastly I removed the free and the calls to sys_brk started showing up.
What is causing this type of behaviour? Does glibc make some kind of optimization in malloc and free functions to prevent useless syscalls?
TL;DR: free followed by malloc makes neither call the kernel. Why?
linux linux-kernel linux-audit
linux linux-kernel linux-audit
edited 56 mins ago
Rui F Ribeiro
41.2k1481139
asked 1 hour ago
Jorge HelenoJorge Heleno
1114
edited 56 mins ago
Rui F Ribeiro
41.2k1481139
asked 1 hour ago
Jorge HelenoJorge Heleno
1114
edited 56 mins ago
Rui F Ribeiro
41.2k1481139
edited 56 mins ago
Rui F Ribeiro
41.2k1481139
edited 56 mins ago
Rui F Ribeiro
41.2k1481139
41.2k1481139
asked 1 hour ago
Jorge HelenoJorge Heleno
1114
asked 1 hour ago
Jorge HelenoJorge Heleno
1114
asked 1 hour ago
Jorge HelenoJorge Heleno
1114
1114
It might use brk () or mmap ()
– 炸鱼薯条德里克
1 hour ago
Neither appeared in strace
– Jorge Heleno
15 mins ago
add a comment |
It might use brk () or mmap ()
– 炸鱼薯条德里克
1 hour ago
Neither appeared in strace
– Jorge Heleno
15 mins ago
It might use brk () or mmap ()
– 炸鱼薯条德里克
1 hour ago
It might use brk () or mmap ()
– 炸鱼薯条德里克
1 hour ago
Neither appeared in strace
– Jorge Heleno
15 mins ago
Neither appeared in strace
– Jorge Heleno
15 mins ago
add a comment |
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It might use brk () or mmap ()
– 炸鱼薯条德里克
1 hour ago
Neither appeared in strace
– Jorge Heleno
15 mins ago