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digitalmars.D.learn - Strange AV in asm mode (code only for amd64)

reply user1234 <user1234 12.nl> writes:
Hello, try this:

---
import std.stdio;

alias Proc = size_t function();

size_t allInnOne()
{
     asm pure nothrow
     {
         mov RAX, 1;
         ret;
         nop;nop;nop;nop;nop;nop;nop;
         mov RAX, 2;
         ret;
     }
}

void main()
{
     Proc proc1 = &allInnOne;
     Proc proc2 = cast(Proc) (cast(void*)proc1 + 16);
     writeln(proc1(), " ", proc2());
}
---

The call to proc1() gens a SEGFAULT after the first RET.
Remove the call to proc1() and it works.

Why that ?
Nov 05
next sibling parent reply user1234 <user1234 12.nl> writes:
On Sunday, 5 November 2017 at 13:43:15 UTC, user1234 wrote:
 [...]
Hmmm it was just the amount of nops. --- import std.stdio; alias Proc = size_t function(); size_t allInnOne() { asm pure nothrow { naked; mov RAX, 1; ret; nop;nop; mov RAX, 2; ret; } } void main() { Proc proc1 = &allInnOne; Proc proc2 = cast(Proc) (cast(void*)&allInnOne + 8); writeln(proc1(), " ",proc2()); } ---
Nov 05
parent user1234 <user1234 12.nl> writes:
On Sunday, 5 November 2017 at 14:25:24 UTC, user1234 wrote:
 On Sunday, 5 November 2017 at 13:43:15 UTC, user1234 wrote:
 [...]
Hmmm it was just the amount of nops. --- import std.stdio; alias Proc = size_t function(); size_t allInnOne() { asm pure nothrow { naked; mov RAX, 1; ret; nop;nop; mov RAX, 2; ret; } } void main() { Proc proc1 = &allInnOne; Proc proc2 = cast(Proc) (cast(void*)&allInnOne + 8); writeln(proc1(), " ",proc2()); } ---
That's a nice trick against static analysis. I imagine well an attacker trying to fight against the first part, even if never executed, assuming he found that this function got executed and then he assumes that the code get executed from the start.
Nov 06
prev sibling parent reply Eugene Wissner <belka caraus.de> writes:
On Sunday, 5 November 2017 at 13:43:15 UTC, user1234 wrote:
 Hello, try this:

 ---
 import std.stdio;

 alias Proc = size_t function();

 size_t allInnOne()
 {
     asm pure nothrow
     {
         mov RAX, 1;
         ret;
         nop;nop;nop;nop;nop;nop;nop;
         mov RAX, 2;
         ret;
     }
 }

 void main()
 {
     Proc proc1 = &allInnOne;
     Proc proc2 = cast(Proc) (cast(void*)proc1 + 16);
     writeln(proc1(), " ", proc2());
 }
 ---

 The call to proc1() gens a SEGFAULT after the first RET.
 Remove the call to proc1() and it works.

 Why that ?
One of the problems is that "naked" is missing in your assembly. If you write asm pure nothrow { naked; mov RAX, 1; ret; nop;nop;nop;nop;nop;nop;nop; mov RAX, 2; ret; } writeln(proc1()) works. Without "naked" dmd generates the prologue and epilogue for your function. Inside the assembly you return from the function without restoring the stack. It causes the segfault. So you have to write the prologue before returning or use nacked assembly. With "naked" and "Proc proc2 = cast(Proc) (cast(void*)proc1 + 8);" the example works.
Nov 05
parent user1234 <user1234 12.nl> writes:
On Sunday, 5 November 2017 at 14:27:18 UTC, Eugene Wissner wrote:
 On Sunday, 5 November 2017 at 13:43:15 UTC, user1234 wrote:
 [...]
One of the problems is that "naked" is missing in your assembly. If you write asm pure nothrow { naked; mov RAX, 1; ret; nop;nop;nop;nop;nop;nop;nop; mov RAX, 2; ret; } writeln(proc1()) works. Without "naked" dmd generates the prologue and epilogue for your function. Inside the assembly you return from the function without restoring the stack. It causes the segfault. So you have to write the prologue before returning or use nacked assembly. With "naked" and "Proc proc2 = cast(Proc) (cast(void*)proc1 + 8);" the example works.
Yeah thanks, i figured it out too. Also there was too much nops.
Nov 05