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  • Doubts in System call mechanism in linux

    - by bala1486
    We transit from ring3 to ring0 using 'int' or the new 'syscall/sysenter' instruction. Does that mean that the page tables and other stuffs that needs to be modified for the kernel is automatically done by the 'int' instruction or the interrupt handler for the 'int 0x80' will do the required stuff and jump to the respective system call. Also when returning from a system call, we again need to go to user space. For this we need to know the instruction address in the user space to continue the user application. Where is that address stored. Does the 'ret' instruction automatically changes the ring from ring3 to ring0 or where/how this ring changing mechanism takes place? Then, i read that changing from ring3 to ring0 is not as costly as changing from ring0 to ring3. Why is this so?? Thanks, Bala

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  • MX Records - go to two servers?

    - by Jim Beam
    Right now I have a single mail server for IMAP. Let's say I want to introduce Exchange but not all users will be on it. Some users will be on my "legacy" IMAP, others on the "new" Exchange. Is it possible to "split up" your users (from the same e-mail domain) on two services like this? What would the MX records look like? My guess is that this isn't possible, but thought I'd ask. By the way, I realize that Exchange can offer IMAP and all that, but my question is more about splitting users across services and the MX records. The actual protocols above are only examples.

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  • Why is a 16-bit register used with BSR instruction in this code snippet?

    - by sharptooth
    In this hardcore article there's a function find_maskwidth() that basically detects the number of bits required to represent itemCount dictinct values: unsigned int find_maskwidth( unsigned int itemCount ) { unsigned int maskWidth, count = itemCount; __asm { mov eax, count mov ecx, 0 mov maskWidth, ecx dec eax bsr cx, ax jz next inc cx mov maskWidth, ecx next: } return maskWidth; } the question is why do they use ax and cx registers instead of eax and ecx?

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  • Segmentation Fault when using "mov" in Assembly

    - by quithakay207
    I am working on a simple assembly program for a class, and am encountering an odd segmentation fault. It's a pretty simple program to convert bytes into kilobytes. However, within the function that does the conversion, I get a segmentation fault when I try to move the value 1024 into the ebx register. I've never had this kind of problem before when working with registers. Does someone know what could be causing this? I imagine it is something simple that I'm overlooking. Thank you! asm_main: enter 0,0 pusha mov eax, 0 mov ebx, 0 call read_int push eax call functionA popa mov leave ret functionA: mov eax, [esp + 4] call print_int call print_nl mov ebx, 1024 ;segmentation fault occurs here div ebx call print_int ret UPDATE: One interesting discovery is that if I delete the lines interacting with the stack, push eax and mov eax, [esp + 4], there is no longer a segmentation fault. However, I get a crazy result in eax after performing div ebx.

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  • SYN flooding still a threat to servers?

    - by Rob
    Well recently I've been reading about different Denial of Service methods. One method that kind of stuck out was SYN flooding. I'm a member of some not-so-nice forums, and someone was selling a python script that would DoS a server using SYN packets with a spoofed IP address. However, if you sent a SYN packet to a server, with a spoofed IP address, the target server would return the SYN/ACK packet to the host that was spoofed. In which case, wouldn't the spoofed host return an RST packet, thus negating the 75 second long-wait, and ultimately failing in its attempt to DoS the server?

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  • getjson and servers

    - by gheil.apl
    Is getJSON only useful for those who control a server? Is there any other alternative for getting data from a file? i tried $.getJSON("good.json", function(data){ out=out+"good.json: "+data + ","; }); where good.json = {"a":"1","b":"2"} and got a result of 'null' for data. These all are valid JSON files, and all give null when used in the above: good.htm assoc.json assoc.js stub.json stub.js test.js test.txt and all get a null result... The above is in an interactive setting at http://jsbin.com/dbJSON/8/edit the output (of null) is to be had by clicking 'output'.

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  • Avoiding cookies while requesting static content

    - by Abdel Olakara
    I just did an audit of one of my web application page (built using ASP.Net and running on development server) using Google chrome's developer tool. One particular warning caught my eyes: Serve static content from a cookieless domain (5)! Here is my screen shot (http://yfrog.com/7eauditresultp) as well. I would like to know is it possible to avoid cookies for these kind of requests. I see that there is no cookie requests for javascript files as well. I it possible to avoid cookies in the header for these files as well? and why didn't the browser attach cookies for javascript files and attach for CSS and image? Any thoughts and suggestions are welcome

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  • How does loop address alignment affect the speed on Intel x86_64?

    - by Alexander Gololobov
    I'm seeing 15% performance degradation of the same C++ code compiled to exactly same machine instructions but located on differently aligned addresses. When my tiny main loop starts at 0x415220 it's faster then when it is at 0x415250. I'm running this on Intel Core2 Duo. I use gcc 4.4.5 on x86_64 Ubuntu. Can anybody explain the cause of slowdown and how I can force gcc to optimally align the loop? Here is the disassembly for both cases with profiler annotation: 415220 576 12.56% |XXXXXXXXXXXXXX 48 c1 eb 08 shr $0x8,%rbx 415224 110 2.40% |XX 0f b6 c3 movzbl %bl,%eax 415227 0.00% | 41 0f b6 04 00 movzbl (%r8,%rax,1),%eax 41522c 40 0.87% | 48 8b 04 c1 mov (%rcx,%rax,8),%rax 415230 806 17.58% |XXXXXXXXXXXXXXXXXXX 4c 63 f8 movslq %eax,%r15 415233 186 4.06% |XXXX 48 c1 e8 20 shr $0x20,%rax 415237 102 2.22% |XX 4c 01 f9 add %r15,%rcx 41523a 414 9.03% |XXXXXXXXXX a8 0f test $0xf,%al 41523c 680 14.83% |XXXXXXXXXXXXXXXX 74 45 je 415283 ::Run(char const*, char const*)+0x4b3 41523e 0.00% | 41 89 c7 mov %eax,%r15d 415241 0.00% | 41 83 e7 01 and $0x1,%r15d 415245 0.00% | 41 83 ff 01 cmp $0x1,%r15d 415249 0.00% | 41 89 c7 mov %eax,%r15d 415250 679 13.05% |XXXXXXXXXXXXXXXX 48 c1 eb 08 shr $0x8,%rbx 415254 124 2.38% |XX 0f b6 c3 movzbl %bl,%eax 415257 0.00% | 41 0f b6 04 00 movzbl (%r8,%rax,1),%eax 41525c 43 0.83% |X 48 8b 04 c1 mov (%rcx,%rax,8),%rax 415260 828 15.91% |XXXXXXXXXXXXXXXXXXX 4c 63 f8 movslq %eax,%r15 415263 388 7.46% |XXXXXXXXX 48 c1 e8 20 shr $0x20,%rax 415267 141 2.71% |XXX 4c 01 f9 add %r15,%rcx 41526a 634 12.18% |XXXXXXXXXXXXXXX a8 0f test $0xf,%al 41526c 749 14.39% |XXXXXXXXXXXXXXXXXX 74 45 je 4152b3 ::Run(char const*, char const*)+0x4c3 41526e 0.00% | 41 89 c7 mov %eax,%r15d 415271 0.00% | 41 83 e7 01 and $0x1,%r15d 415275 0.00% | 41 83 ff 01 cmp $0x1,%r15d 415279 0.00% | 41 89 c7 mov %eax,%r15d

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  • Uploading file to multiple servers ?

    - by WarDoGG
    I'm creating an image uploader which will give the user an option to upload file to a different server than which the script is hosted on. Is making an FTP connection from the script server to the target uploading server the only way to do this ? Correct me if im wrong, but would the path of the file look like this ? User computer (file) --- script server ---- target server Is there another way to do this ?

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  • Problem with asm program (nasm)

    - by GLeBaTi
    org 0x100 SEGMENT .CODE mov ah,0x9 mov dx, Msg1 int 0x21 ;string input mov ah,0xA mov dx,buff int 0x21 mov ax,0 mov al,[buff+1]; length ;string UPPERCASE mov cl, al mov si, buff cld loop1: lodsb; cmp al, 'a' jnb upper loop loop1 ;output mov ah,0x9 mov dx, buff int 0x21 exit: mov ah, 0x8 int 0x21 int 0x20 upper: sub al,32 jmp loop1 SEGMENT .DATA Msg1 db 'Press string: $' buff db 254,0 this code perform poorly. I think that problem in "jnb upper". This program make small symbols into big symbols.

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  • copy files between servers asp.net mvc

    - by kalyan
    Hi, I am using asp.net, c#, MVC and nHibernate and I am trying to upload a file from a local machine to the server and replicate the file to the different server. I was able to upload file to the server and copy the file from one folder to the other folder on the same server without any problem.But how can I copy the file from one server to another server. Please follow the link to see how to copy a file from one folder to another folder on the same server. Click to see my answer to the file upload question.[please look for answer by kalyan] Please help. Thank you.

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  • help understanding differences between #define, const and enum in C and C++ on assembly level.

    - by martin
    recently, i am looking into assembly codes for #define, const and enum: C codes(#define): 3 #define pi 3 4 int main(void) 5 { 6 int a,r=1; 7 a=2*pi*r; 8 return 0; 9 } assembly codes(for line 6 and 7 in c codes) generated by GCC: 6 mov $0x1, -0x4(%ebp) 7 mov -0x4(%ebp), %edx 7 mov %edx, %eax 7 add %eax, %eax 7 add %edx, %eax 7 add %eax, %eax 7 mov %eax, -0x8(%ebp) C codes(enum): 2 int main(void) 3 { 4 int a,r=1; 5 enum{pi=3}; 6 a=2*pi*r; 7 return 0; 8 } assembly codes(for line 4 and 6 in c codes) generated by GCC: 6 mov $0x1, -0x4(%ebp) 7 mov -0x4(%ebp), %edx 7 mov %edx, %eax 7 add %eax, %eax 7 add %edx, %eax 7 add %eax, %eax 7 mov %eax, -0x8(%ebp) C codes(const): 4 int main(void) 5 { 6 int a,r=1; 7 const int pi=3; 8 a=2*pi*r; 9 return 0; 10 } assembly codes(for line 7 and 8 in c codes) generated by GCC: 6 movl $0x3, -0x8(%ebp) 7 movl $0x3, -0x4(%ebp) 8 mov -0x4(%ebp), %eax 8 add %eax, %eax 8 imul -0x8(%ebp), %eax 8 mov %eax, 0xc(%ebp) i found that use #define and enum, the assembly codes are the same. The compiler use 3 add instructions to perform multiplication. However, when use const, imul instruction is used. Anyone knows the reason behind that?

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  • Setting processor to 32-bit mode

    - by dboarman-FissureStudios
    It seems that the following is a common method given in many tutorials on switching a processor from 16-bit to 32-bit: mov eax, cr0 ; set bit 0 in CR0-go to pmode or eax, 1 mov cr0, eax Why wouldn't I simply do the following: or cr0, 1 Is there something I'm missing? Possibly the only thing I can think of is that I cannot perform an operation like this on the cr0 register.

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  • Using mail servers in java

    - by sword101
    greeting all i want to use a mail server where the users send emails to it and then i parse this emails then do some action please suggest me what mail server to be used and where to start some links,tutorials,guide is very appreciated .

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  • How to scale a PHP application (servers, mysql, memcache)

    - by Stéphane Goetz
    Hi, I'm currently creating a website for a social project in switzerland. And before there is an overflow of user, I want to prepare the application to scale. I answered by myself many questions but some are left. I explain what I want to do. First at the beginnning, the Application will have only one server (short time) with DNS, PHP, Mysql, Data, and memcache. Second Then I will split them in two DNS, Mysql, memcache Data, PHP Third Here is the problem, I don't know how to do it exactly here to keep the application running well. I could do : Front : Load Balancer, memcache, DNS Web 1 : PHP, DATA Web 2 : PHP, DATA Mysql This would be the scheme, all PHP sessions are kept in the DB. BUT, how do I sync the data? do I run a Rsync to keep them up to date. do I put them on a separate disk (network disk) to be sure ? but in this case, how can I do in case of user uploads ? and if the website gets more success and we have to go on greater structures, would'nt it create some latency on updates ? or would it be a good thing to go directly to amazon's web services ? some infos I use codeigniter as Framework. I use linux as webserver (distribution not chosen now, but should be Debian) Thanks in advance for your answers.

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  • Using delayed_job to process file uploads across multiple servers

    - by Steve Klabnik
    Does anyone have any good resources on how to do this? Basically, I'm working on a project (in Rails) where people can upload files. They might be big. I'd like to process them using delayed_job before sending them to S3. I'd also like to do this processing on a separate job queue server, rather than on the webserver itself. I'd rather not have to upload the files to the webserver, then transfer them to the job queue server, and then upload them to S3 if I don't have to. Thanks.

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  • Why is FLD1 loading NaN instead?

    - by Bernd Jendrissek
    I have a one-liner C function that is just return value * pow(1.+rate, -delay); - it discounts a future value to a present value. The interesting part of the disassembly is 0x080555b9 : neg %eax 0x080555bb : push %eax 0x080555bc : fildl (%esp) 0x080555bf : lea 0x4(%esp),%esp 0x080555c3 : fldl 0xfffffff0(%ebp) 0x080555c6 : fld1 0x080555c8 : faddp %st,%st(1) 0x080555ca : fxch %st(1) 0x080555cc : fstpl 0x8(%esp) 0x080555d0 : fstpl (%esp) 0x080555d3 : call 0x8051ce0 0x080555d8 : fmull 0xfffffff8(%ebp) While single-stepping through this function, gdb says (rate is 0.02, delay is 2; you can see them on the stack): (gdb) si 0x080555c6 30 return value * pow(1.+rate, -delay); (gdb) info float R7: Valid 0x4004a6c28f5c28f5c000 +41.68999999999999773 R6: Valid 0x4004e15c28f5c28f6000 +56.34000000000000341 R5: Valid 0x4004dceb851eb851e800 +55.22999999999999687 R4: Valid 0xc0008000000000000000 -2 =R3: Valid 0x3ff9a3d70a3d70a3d800 +0.02000000000000000042 R2: Valid 0x4004ff147ae147ae1800 +63.77000000000000313 R1: Valid 0x4004e17ae147ae147800 +56.36999999999999744 R0: Valid 0x4004efb851eb851eb800 +59.92999999999999972 Status Word: 0x1861 IE PE SF TOP: 3 Control Word: 0x037f IM DM ZM OM UM PM PC: Extended Precision (64-bits) RC: Round to nearest Tag Word: 0x0000 Instruction Pointer: 0x73:0x080555c3 Operand Pointer: 0x7b:0xbff41d78 Opcode: 0xdd45 And after the fld1: (gdb) si 0x080555c8 30 return value * pow(1.+rate, -delay); (gdb) info float R7: Valid 0x4004a6c28f5c28f5c000 +41.68999999999999773 R6: Valid 0x4004e15c28f5c28f6000 +56.34000000000000341 R5: Valid 0x4004dceb851eb851e800 +55.22999999999999687 R4: Valid 0xc0008000000000000000 -2 R3: Valid 0x3ff9a3d70a3d70a3d800 +0.02000000000000000042 =R2: Special 0xffffc000000000000000 Real Indefinite (QNaN) R1: Valid 0x4004e17ae147ae147800 +56.36999999999999744 R0: Valid 0x4004efb851eb851eb800 +59.92999999999999972 Status Word: 0x1261 IE PE SF C1 TOP: 2 Control Word: 0x037f IM DM ZM OM UM PM PC: Extended Precision (64-bits) RC: Round to nearest Tag Word: 0x0020 Instruction Pointer: 0x73:0x080555c6 Operand Pointer: 0x7b:0xbff41d78 Opcode: 0xd9e8 After this, everything goes to hell. Things get grossly over or undervalued, so even if there were no other bugs in my freeciv AI attempt, it would choose all the wrong strategies. Like sending the whole army to the arctic. (Sigh, if only I were getting that far.) I must be missing something obvious, or getting blinded by something, because I can't believe that fld1 should ever possibly fail. Even less that it should fail only after a handful of passes through this function. On earlier passes the FPU correctly loads 1 into ST(0). The bytes at 0x080555c6 definitely encode fld1 - checked with x/... on the running process. What gives?

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  • use callback function to report stack backtrace

    - by user353394
    Assume I have the following: typedef struct { char *name; char binding; int address; } Fn_Symbol //definition of function symbol static Fn_Symbol *fnSymbols; //array of function symbols in a file statc int total; //number of symbol functions in the array and file static void PrintBacktrace(int sigum, siginfo_t * siginfo, void *context) { printf("\nSignal received %d (%s)\n", signum, strsignal(signum)); const int eip_index = 14; void *eip = (void *)((struct ucontext *)context)->uc_mcontext.gregs[eip_index]; printf("Error at [%p] %s (+0x%x), eip, fnName, offset from start); //????? exit(0); } I have this so far, but what is the best way using the fnSymbols static global pointer to identify the function where the error occured and then back trace through the stack to identify each calling function by address, name, and offset?

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  • Ajax request (from .NET) give me unexpected results

    - by ironnailpiercethesky
    I wanted to learn a little more about web and .NET so i decided to write a program to download binaries, webpages and does a little ajax. The first download site i tried was too easy. So i went against uploading.com. It seemed good, no captcha, some ajax use required, you cant guess all the data, it was perfect. After 2hours of messing around i got frustrated. For the life of me i could not get the ajax response to give me what i was expecting. I decided to use tamper data clear my cookies, click on the entry for the ajax request. Copy the Referer, cookies and postdata and came up with this code. It does not work! I am shocked that copying those fields did not work. I even tried setting the user agent (not in code linked) and it still did not work. Why isnt this POST on the ajax request not giving me the correct file? I notice the JsHttpRequest=12719132559480-xml changes based on time so if you want to run the code you should grab a new snapshot with tamperdata (or at least update that url). I dont see any extra headers in tamper data so does anyone know why i might not be getting the response expected? I know disabling cookies or javascript will make the link 'expired' automatically. I dont think javascript will matter because i got the data from tamperdata at the moment of request so what gives? What am i forgetting or what data am i not submitting properly?

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  • Odd optimization problem under MSVC

    - by Goz
    I've seen this blog: http://igoro.com/archive/gallery-of-processor-cache-effects/ The "weirdness" in part 7 is what caught my interest. My first thought was "Thats just C# being weird". Its not I wrote the following C++ code. volatile int* p = (volatile int*)_aligned_malloc( sizeof( int ) * 8, 64 ); memset( (void*)p, 0, sizeof( int ) * 8 ); double dStart = t.GetTime(); for (int i = 0; i < 200000000; i++) { //p[0]++;p[1]++;p[2]++;p[3]++; // Option 1 //p[0]++;p[2]++;p[4]++;p[6]++; // Option 2 p[0]++;p[2]++; // Option 3 } double dTime = t.GetTime() - dStart; The timing I get on my 2.4 Ghz Core 2 Quad go as follows: Option 1 = ~8 cycles per loop. Option 2 = ~4 cycles per loop. Option 3 = ~6 cycles per loop. Now This is confusing. My reasoning behind the difference comes down to the cache write latency (3 cycles) on my chip and an assumption that the cache has a 128-bit write port (This is pure guess work on my part). On that basis in Option 1: It will increment p[0] (1 cycle) then increment p[2] (1 cycle) then it has to wait 1 cycle (for cache) then p[1] (1 cycle) then wait 1 cycle (for cache) then p[3] (1 cycle). Finally 2 cycles for increment and jump (Though its usually implemented as decrement and jump). This gives a total of 8 cycles. In Option 2: It can increment p[0] and p[4] in one cycle then increment p[2] and p[6] in another cycle. Then 2 cycles for subtract and jump. No waits needed on cache. Total 4 cycles. In option 3: It can increment p[0] then has to wait 2 cycles then increment p[2] then subtract and jump. The problem is if you set case 3 to increment p[0] and p[4] it STILL takes 6 cycles (which kinda blows my 128-bit read/write port out of the water). So ... can anyone tell me what the hell is going on here? Why DOES case 3 take longer? Also I'd love to know what I've got wrong in my thinking above, as i obviously have something wrong! Any ideas would be much appreciated! :) It'd also be interesting to see how GCC or any other compiler copes with it as well! Edit: Jerry Coffin's idea gave me some thoughts. I've done some more tests (on a different machine so forgive the change in timings) with and without nops and with different counts of nops case 2 - 0.46 00401ABD jne (401AB0h) 0 nops - 0.68 00401AB7 jne (401AB0h) 1 nop - 0.61 00401AB8 jne (401AB0h) 2 nops - 0.636 00401AB9 jne (401AB0h) 3 nops - 0.632 00401ABA jne (401AB0h) 4 nops - 0.66 00401ABB jne (401AB0h) 5 nops - 0.52 00401ABC jne (401AB0h) 6 nops - 0.46 00401ABD jne (401AB0h) 7 nops - 0.46 00401ABE jne (401AB0h) 8 nops - 0.46 00401ABF jne (401AB0h) 9 nops - 0.55 00401AC0 jne (401AB0h) I've included the jump statetements so you can see that the source and destination are in one cache line. You can also see that we start to get a difference when we are 13 bytes or more apart. Until we hit 16 ... then it all goes wrong. So Jerry isn't right (though his suggestion DOES help a bit), however something IS going on. I'm more and more intrigued to try and figure out what it is now. It does appear to be more some sort of memory alignment oddity rather than some sort of instruction throughput oddity. Anyone want to explain this for an inquisitive mind? :D Edit 3: Interjay has a point on the unrolling that blows the previous edit out of the water. With an unrolled loop the performance does not improve. You need to add a nop in to make the gap between jump source and destination the same as for my good nop count above. Performance still sucks. Its interesting that I need 6 nops to improve performance though. I wonder how many nops the processor can issue per cycle? If its 3 then that account for the cache write latency ... But, if thats it, why is the latency occurring? Curiouser and curiouser ...

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  • How do you use printf from Assembly?

    - by bobobobo
    I have an MSVC++ project set up to compile and run assembly code. In main.c: #include <stdio.h> void go() ; int main() { go() ; // call the asm routine } In go.asm: .586 .model flat, c .code go PROC invoke puts,"hi" RET go ENDP end But when I compile and run, I get an error in go.asm: error A2006: undefined symbol : puts How do I define the symbols in <stdio.h> for the .asm files in the project?

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