For a user-lever thread library, I need to figure out jumping to a function by modifying PC value stored in jmp_buf.
This is what I have written:
jmp_buf env;
void print (void) {
printf("\nHello World!");
}
static int ptr_mangle(int p) {
unsigned int ret;
asm(" movl %1, %%eax;\n"
" xorl %%gs:0x18, %%eax;"
" roll $0x9, %%eax;"
" movl %%eax, %0;"
: "=r"(ret)
: "r"(p)
: "%eax"
);
return ret;
}
int main() {
int i = setjmp(env);
env[0].__jmpbuf[5] = ptr_mangle(print);
longjmp(env, 2);
return 0;
}
I am trying to modify PC in jmp_buf by setting it to the address of the function I am trying to jump to.
I am getting a segmentation fault.
I am unable to figure out what exactly needs to be done. Do I need to modify SP as well?
Any help would be very much appreciated.
What are you trying to do? Are you not checking for the return value of setjmp? I don't think you are doing this correctly. Have a look at the sample code below to see what would be the output be:
#include <stdio.h>
#include <setjmp.h>
#include <stdlib.h>
void subroutine(jmp_buf);
int main(void)
{
int value;
jmp_buf jumper;
value = setjmp(jumper);
if (value != 0)
{
printf("Longjmp with value %d\n", value);
exit(value);
}
printf("About to call subroutine ... \n");
subroutine(jumper);
return 0;
}
void subroutine(jmp_buf jumper)
{
longjmp(jumper,1);
}
The output would be:
About to call subroutine...
Longjmp with a value of 1.
Which begs the question - why are you trying to modify the IP? It sounds like you overwrote something or the code 'jumped' off into the woods and trampled something and came back with a hard landing i.e. segfault.
The variable env is specifically a struct, do not use an array subscript as you have done. I suspect that is why you got a segfault...
Hope this helps,
Best regards,
Tom.
Related
I'm new to llvm , and was trying to find lock declaration statement and then do some instrumention work,the code like this:
#include <iostream>
#include <thread>
#include <mutex>
using namespace std;
int share = 42;
mutex m;
void f()
{
m.lock();
--share;
cout << "function f -> share: " << share << '\n';
m.unlock();
}
int main()
{
thread thf{f};
thf.join();
return 0;
}
I want to find the lock declaration instruction eg:
mutex m;
the llvm instrumention pass like this:
struct SkeletonPass : public FunctionPass {
static char ID;
SkeletonPass() : FunctionPass(ID) {}
virtual bool runOnFunction(Function &F) {
// Get the function to call from our runtime library.
LLVMContext &Ctx = F.getContext();
Constant *logFunc = F.getParent()->getOrInsertFunction(
"logop", Type::getVoidTy(Ctx), Type::getInt32Ty(Ctx), NULL
);
for (auto &B : F) {
for (auto &I : B) {
***if ((&I) is lock declaration instruction)*** {
// Insert something *after* `op`.
IRBuilder<> builder(op);
builder.SetInsertPoint(&B, ++builder.GetInsertPoint());
// Insert a call to function.
builder.CreateCall(logFunc, ConstantInt::get(Type::getInt32Ty(Ctx), 2));
return true;
}
}
}
In short, could you please tell me how to discover lock declaration instruction, thanks!
The declaration would appear as a global, so you should write a module pass to find it, not a function pass. It should appear as something like:
#m = global %mutex zeroinitializer
In fact, using the demo at http://ellcc.org/demo/index.cgi to try this, you can indeed see that:
...
%"class.std::__1::mutex" = type { %struct.pthread_mutex_t }
%struct.pthread_mutex_t = type { %union.anon }
%union.anon = type { [5 x i8*] }
...
#m = global %"class.std::__1::mutex" zeroinitializer, align 8
You can use LLVM's CppBackend to compile your code. This would produce a C++ code that makes up the source. You can then easily find out how mutex m; definition is constructed via LLVM API.
Run clang -march=cpp foo.cpp to use CppBackend. Alternatively, you can use this demo page to compile your code online.
You can write the prototypes without the variable names?
int example(examplestruct *var1, examplestruct *var2);
void done(examplestruct *var1,FILE *f);
struct {
int* field1;
int field2;
}examplestruct;
Is it possible to write the prototypes without name variables?
Can anyone tell me if this is acceptable in C language? You can write the prototypes without the variable names?
Yes.
As for the second question:
If you want a function to be inside main(), then take the body of the function, put it in main() and make sure that the arguments that the function had are well handled.
This example will clear things up:
#include <stdio.h>
void print(int);
void inc_p(int);
int main(void) {
int num = 5;
print(num);
inc_p(num);
// to get rid of inc_p(), copy paste it's body inside main
// and you will get this
// a++;
// print(a);
// However, a was an argument, here you need to use
// the variable declared in main(), i.e. 'num'
num++;
print(num);
return 0;
}
void print(int a) {
printf("%d\n", a);
}
void inc_p(int a) {
a++;
print(a);
}
We know that we call pthread like this:
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
void *(*start_routine) (void *), void* arg);
Hi guys, i want to know why the return type of third parameter is void*? why not void?
Because there is no way for a start function to know what kind of data a developer wants to return from the function they use a void* that can point to any type. It is up to the developer of the start function to then cast the void* to appropriate type he actually returned before using whatever the void* points to. So now the start function can return a pointer that may in actually point to anything. If the start function is declared to return void, it means this function returns nothing, then what if the developer wants the start function to return a int, a struct? For example:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <pthread.h>
struct test {
char str[32];
int x;
};
void *func(void*) {
struct test *eg = (struct test *)malloc(sizeof(struct test));
strcpy(eg->str,"hello world");
eg->x = 42;
pthread_exit(eg);
}
int main (void) {
pthread_t id;
struct test *resp;
pthread_create(&id, NULL, func, NULL);
pthread_join(id,(void**)&resp);
printf("%s %d\n",resp->str,resp->x);
free(resp);
return 0;
}
More details on this post: What does void* mean and how to use it?
I am not able to execute pthreads program in c. Please tell me what is wrong with the following program. I am neither getting any error nor expected output.
void *worker(void * arg)
{
int i;
int *id=(int *)arg;
printf("Thread %d starts\n", *id );
}
void main(int argc, char **argv)
{
int thrd_no,i,*thrd_id,rank=0;
void *exit_status;
pthread_t *threads;
thrd_no=atoi(argv[1]-1);
thrd_id= malloc(sizeof(int)*(thrd_no));
threads=malloc(sizeof(pthread_t)*(thrd_no));
for(i=0;i<thrd_no;i++)
{
rank=i+1;
thrd_id[i]=pthread_create(&threads[i], NULL, worker, &rank);
}
for(i=0;i<thrd_no;i++)
{
pthread_join(threads[i], &exit_status);
}
}
thrd_no = atoi(argv[1] - 1); likely doesn't do what you intended; the way argv is normally passed into a new process and parsed into a C array, argv[1] - 1 is probably pointing at \0 (specifically, the \0 at the end of argv[0]). (More generally, indexing backwards off the start of a string is rarely correct.) The result is that atoi() will return 0 and no threads will be created. What did you actually intend to do there?
You are passing the same address &rank to each thread, so id and *id is the same for all your worker-s.
You should better allocate on the heap the address you pass to each worker routine.
You might also include <stdint.h and use intptr_t, e.g.
void worker (void* p)
{
intptr_t rk = (intptr_t) p;
/// etc
}
and call
intptr_t rank = i + 1;
thrd_id[i]=pthread_create(&threads[i], NULL, worker, (void*)rank);
You should learn to use a debugger and compile with all warnings and debug information, i.e. gcc -Wall -g (and improve your code till it gets no warnings, then use gdb)
code segment rank=i+1;
thrd_id[i]=pthread_create(&threads[i], NULL, worker, &rank);
will produce race condition.
I'm newer to C++. I have written some code, but when i run it, there's always this:
raised exception class
EAccessViolation with message 'Access
violation at address'
i don't understand this. Would you like to help me solve it? It's important to me. Really, really thank you!
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <math.h>
#include <conio.h>
#define k 2
#define minoffset 0.5
using namespace std;
struct Point
{
double X;
double Y;
};
vector<Point> dataprocess();
void k_means(vector<Point> points,int N);
double getdistance(Point p1,Point p2)
{ double distance;
distance=sqrt((p1.X-p2.X)*(p1.X-p2.X)+(p1.Y-p2.Y)*(p1.Y-p2.Y));
return distance;
}
int getmindis(Point p,Point means[])
{
int i;
int c;
double dis=getdistance(p,means[0]);
for(i=1;i<k;i++)
{
double term=getdistance(p,means[i]);
if(term<dis)
{
c=i;
dis=term;
}
}
return c;
}
Point getmeans(vector<Point> points)
{
int i;
double sumX,sumY;
Point p;
int M=points.size();
for(i=0;i<M;i++)
{
sumX=points[i].X;
sumY=points[i].Y;
}
p.X=sumX/M;
p.Y=sumY/M;
return p;
}
int main()
{ int N;
vector<Point> stars;
stars=dataprocess();
N=stars.size();
cout<<"the size is:"<<N<<endl;
k_means(stars,N);
getch();
}
vector<Point> dataprocess()
{
int i;
int N;
double x,y;
vector<Point> points;
Point p;
string import_file;
cout<<"input the filename:"<<endl;
cin>>import_file;
ifstream infile(import_file.c_str());
if(!infile)
{
cout<<"read error!"<<endl;
}
else
{
while(infile>>x>>y)
{
p.X=x;
p.Y=y;
points.push_back(p);
}
}
N=points.size();
cout<<"output the file data:"<<endl;
for(i=0;i<N;i++)
{
cout<<"the point"<<i+1<<"is:X="<<points[i].X<<" Y="<<points[i].Y<<endl;
}
return points;
}
void k_means(vector<Point> points,int N)
{
int i;
int j;
int index;
vector<Point> clusters[k];
Point means[k];
Point newmeans[k];
double d,offset=0;
bool flag=1;
cout<<"there will be"<<k<<"clusters,input the original means:"<<endl;
for(i=0;i<k;i++)
{
cout<<"k"<<i+1<<":"<<endl;
cin>>means[i].X>>means[i].Y;
}
while(flag)
{
for(i=0;i<N;i++)
{
index=getmindis(points[i],means);
clusters[index].push_back(points[i]);
}
for(j=0;j<k;j++)
{
newmeans[j]=getmeans(clusters[j]);
offset=getdistance(newmeans[j],means[j]);
}
if(offset>d)
{
d=offset;
}
flag=(minoffset<d)?true:false;
for(i=0;i<k;i++)
{
means[i]=newmeans[i];
clusters[i].clear();
}
}
for(i=0;i<k;i++)
{
cout<<"N"<<i+1<<"="<<clusters[i].size()<<endl;
cout<<"the center of k"<<i+1<<"is:"<<means[i].X<<" "<<means[i].Y<< endl;
}
}
You surely have some algo errors in you code. It is difficult to deal with code without input data, that caused an error, but let's try:
First, lets look at function Point getmeans(vector<Point> points)
it is supposed to evaluate mean coordinates for cluster of points: if you pass an empty cluster to this function it will cause an error:
look here -
int M=points.size()
and here -
for(i=0;i<M;i++)
{
sumX=points[i].X;
sumY=points[i].Y;
}
if your cluster is empty than M will be zero and you loop will iterate 2^31 times (until 32 bit integer overflow) and each time you will try to read values of nonexistent vector items
So, You have to test if you vector is not empty before running main function loop and you have to decide which mean values should be assigned for zero cluster (May be you need an additional flag for empty cluster which will be checked before dealing with cluster's mean values)
Then lets examine function int getmindis(Point p,Point means[]) and, also, a place, where we call it:
index=getmindis(points[i],means); clusters[index].push_back(points[i]);
This function assings points to clusters. cluster number is ruled by c variable. If input point doesn't fit to any cluster, function will return uninitialized variable (holding any possible value) which. then is used as vector index of nonexisting element - possible access violation error
You probably have to initialize c to zero in declaration
Tell us when you will be ready with errors described above and also show us a sample input file (one which causes errors, if all datasets cause errors, show us the smallest one)