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He claimed his code work on Linux but didn't work when he ported to Cygwin. He's writing a Scheme interpreter which would aim for portability one assumes. Whe

by ww520 16y ago

He claimed his code work on Linux but didn't work when he ported to Cygwin. He's writing a Scheme interpreter which would aim for portability one assumes. Where did he say he's doing a non-portable exclusively Cygwin implementation? Stop putting your words into his mouth to make your argument. I specifically wrote the return address of main() was messed up in his code and that was what not working. Please point out what is wrong with that assessment. You are bringing up far fetched ideas and arguments that I didn't mention. Stop putting words in my mouth. Ok, I made a mistake with the register name. It's ESS:ESP instead of EBP:ESP. I meant he forgot to set up the stack segment register. The stack segment is most likely different from the heap segment. Switching the ESP but not the ESS is asking for trouble. I will NOT tell you how to implement a portable native-stack. That's not my intention. What I have done is pointing out exactly what were not working in his code. You are the one failing on reading comprehension and making wild assumption and arguing in different directions.

tptacek 16y ago
There is no "ESS" register on x86. You've confused the stack segment selector, which is an addressing format quirk (and which virtually never changes), with the register file. Please, stop digging.
- ww520 16y ago
  Read item 1 under 2.3.3 Stack Implementation in the Intel 80386 Reference Programmer's Manual. http://pdos.csail.mit.edu/6.828/2006/readings/i386/s02_03.htm http://pdos.csail.mit.edu/6.828/2006/readings/i386/s02_03.ht...
  - tptacek 16y ago
    The stack segment descriptor virtually never changes in userland protected mode code. You're not "asking for trouble" by not fucking with it. It's also not simply a pointer, unlike EBP with frame pointers. And it's called SS, not "ESS", and it's implied in stack operations.
    - ww520 16y ago
      Read 2.3.2 Segment Registers.
      - tptacek 16y ago
        Have you ever changed a segment register in userland code? Tell me about the code that did it and why. And while you're at it: have you ever written code that switched stacks? You might have had to if you wrote a userland threading library, or if you wrote an exception handling system, or a coroutine library. This stuff you're critiquing this guy on the Cygwin list about: are you an expert? I saw your Reddit comments; I believe you're a competent WinAPI C programmer. What I want to know is, do you actually know assembly? Or just have a lot of opinions about it?
        ww520 16y ago
        Yes, I had, in both user mode or kernel mode codes. Where do you think I got the idea about stack segment register? I don't need to prove how awesome an expert I am, unlike someone. My comments remain as they stand. You can insist until your face turns blue that you've never seen a stack segment register changed. The fact remains the Intel standard reference states that the stack segment register can be changed and is used in stack operations. The difference between you and me is that I code defensively to handle the published standard contract so as not to worry about some corner cases coming up in some compiler implementation while you ignore the published standard and base decision solely on what you have seen. That's fine, to each their own. I just find the cavalier attitude surprising coming from a supposed security expert. It's obvious that you don't have anything constructive to say and arguing purely for argument sake. That's the end of this discussion.
        tptacek 16y ago
        I asked if you'd ever changed a segment selector in userland code and why. You said "yes", didn't say why, blew smoke about how "Intel says its possible to change them!" (no shit?), said changing the stack segment selector is "defensive coding" (again: userland code practically disables segment addressing, except for conveniences like the TEB at GS), made reference to a "published standard", and then called me a "supposed" security expert. And, let's be clear, you're the expert that can't remember the eight (8) GPRs on x86. I'm convinced you can code, but I'm equally convinced that you had no business berating someone else for asking questions about how Cygwin handles its stack, because you don't know either. Don't pile on to people.

by ww520 16y ago

tptacek 16y ago

There is no "ESS" register on x86. You've confused the stack segment selector, which is an addressing format quirk (and which virtually never changes), with the register file. Please, stop digging.

ww520 16y ago
Read item 1 under 2.3.3 Stack Implementation in the Intel 80386 Reference Programmer's Manual. http://pdos.csail.mit.edu/6.828/2006/readings/i386/s02_03.htm http://pdos.csail.mit.edu/6.828/2006/readings/i386/s02_03.ht...
- tptacek 16y ago
  The stack segment descriptor virtually never changes in userland protected mode code. You're not "asking for trouble" by not fucking with it. It's also not simply a pointer, unlike EBP with frame pointers. And it's called SS, not "ESS", and it's implied in stack operations.
  - ww520 16y ago
    Read 2.3.2 Segment Registers.
    - tptacek 16y ago
      Have you ever changed a segment register in userland code? Tell me about the code that did it and why. And while you're at it: have you ever written code that switched stacks? You might have had to if you wrote a userland threading library, or if you wrote an exception handling system, or a coroutine library. This stuff you're critiquing this guy on the Cygwin list about: are you an expert? I saw your Reddit comments; I believe you're a competent WinAPI C programmer. What I want to know is, do you actually know assembly? Or just have a lot of opinions about it?
      - ww520 16y ago
        Yes, I had, in both user mode or kernel mode codes. Where do you think I got the idea about stack segment register? I don't need to prove how awesome an expert I am, unlike someone. My comments remain as they stand. You can insist until your face turns blue that you've never seen a stack segment register changed. The fact remains the Intel standard reference states that the stack segment register can be changed and is used in stack operations. The difference between you and me is that I code defensively to handle the published standard contract so as not to worry about some corner cases coming up in some compiler implementation while you ignore the published standard and base decision solely on what you have seen. That's fine, to each their own. I just find the cavalier attitude surprising coming from a supposed security expert. It's obvious that you don't have anything constructive to say and arguing purely for argument sake. That's the end of this discussion.
        tptacek 16y ago
        I asked if you'd ever changed a segment selector in userland code and why. You said "yes", didn't say why, blew smoke about how "Intel says its possible to change them!" (no shit?), said changing the stack segment selector is "defensive coding" (again: userland code practically disables segment addressing, except for conveniences like the TEB at GS), made reference to a "published standard", and then called me a "supposed" security expert. And, let's be clear, you're the expert that can't remember the eight (8) GPRs on x86. I'm convinced you can code, but I'm equally convinced that you had no business berating someone else for asking questions about how Cygwin handles its stack, because you don't know either. Don't pile on to people.