The Linux Backdoor Attempt of 2003 (2013)

Post by simias » Mon, Aug 10, 2020, 8:30 PM UTC

kazinator wrote: post #90

simias wrote: post #29

Earlier quoted context omitted.

I would put parentheses here, I never like mixing logical operators with other types (or even different types of logical operators). While it's of course entirely redundant here, it also makes the code easier to read IMO. I think the parent's point is more convincing: why make this check only for root in the first place?

The parentheses are required if == is changed to =. == has a higher precedence than &&, but = has a lower precedence. a = b && c && d means a = (b && c && d)

    let invalid_options = (options == (__WCLONE|__WALL));
    let is_root = (current->uid == 0);

    if invalid_options && is_root {
        // ...
    }

Post by GuB-42 » Mon, Aug 10, 2020, 9:19 PM UTC

a1369209993 wrote: post #127

GuB-42 wrote: post #105

Earlier quoted context omitted.

It would break these kinds of constructs, which are common. if ((fd = open(...)) != -1) { /* do something with fd */ } else { perror("open"); } The compiler outputs a warning when you have something like "if (a = b)". If that's what you mean (it sometimes is), you have to write it "if ((a = b))" to silence the warning.

TBF, that doesn't actually work - you have to write: int fd; /* fd must be declared in order to be assigned */ if ((fd = open(...)) != -1) { /* do something with fd */ } else { perror("open"); } which would be better written as: int fd = open(...); if (fd != -1) /* etc */ It would be nice for scoping reasons to be able to write something like: if ((int fd = open(...)) != -1) /* etc */ but if ((int fd == open(...)) !=…

  for (int fd = open(...); fd != -1; fd = -1) {
      /* do stuff */
  }

Post by flingo » Mon, Aug 10, 2020, 10:54 PM UTC

db48x wrote: post #123

flingo wrote: post #115

Earlier quoted context omitted.

Why would it require that much data? I always thought you wouldn't need to add or change more bytes than are in the output. Also, git hashes aren't just based on source code. You can add that data anywhere that git uses to generate the hash.

That's true of a CRC code, but hashes are a lot harder to break. Git hashes each file, and puts those hashes into a tree object, like a directory listing. Then it hashes the trees, recursively back up to the root of the repository. Finally the hash of the root tree is put in the commit object, and the commit object is hashed. Thus the two places you can put additional data to be hashed are the file contents (either i…

Post by saagarjha » Tue, Aug 11, 2020, 12:10 AM UTC

GuB-42 wrote: post #105

mhh__ wrote: post #103

Earlier quoted context omitted.

It should really be forbidden by the compiler these days, or at least a very loud warning.

It would break these kinds of constructs, which are common. if ((fd = open(...)) != -1) { /* do something with fd */ } else { perror("open"); } The compiler outputs a warning when you have something like "if (a = b)". If that's what you mean (it sometimes is), you have to write it "if ((a = b))" to silence the warning.

  char *strcpy(char *d, char *s) {
      char *r = d;
      while (*d++ = *s++);
      return r;
  }

Post by a1369209993 » Tue, Aug 11, 2020, 3:08 AM UTC

GuB-42 wrote: post #133

a1369209993 wrote: post #127

Earlier quoted context omitted.

TBF, that doesn't actually work - you have to write: int fd; /* fd must be declared in order to be assigned */ if ((fd = open(...)) != -1) { /* do something with fd */ } else { perror("open"); } which would be better written as: int fd = open(...); if (fd != -1) /* etc */ It would be nice for scoping reasons to be able to write something like: if ((int fd = open(...)) != -1) /* etc */ but if ((int fd == open(...)) !=…

Exactly my thoughts, I was about to comment on that but I was too lazy so I omitted the declaration. Obviously, this is HN, so someone had to point it out ;) Anyways, I am a bit torn about the second option. I like the idea of putting the call inside the if clause as it makes for a very explicit error handling but the uninitialized declaration is ugly. What I do in practice tends to depend on the situation but it is…

  > for (int fd = open(...); fd != -1; fd = -1) {

  #define LET(...) for(__VA_ARGS__,_tmp[0],*_once=_tmp; _once ;_once=0)
  /* ^^^ goes in a header file */
  LET(int fd = open())
    {
    if(fd == -1) { /* handle error, return or break */ }
    /* do stuff */
    }