What Every C Programmer Should Know About Undefined Behavior #1/3

What Every C Programmer Should Know About Undefined Behavior #1/3(blog.llvm.org)

155 points by zeugma 15 years ago | 18 comments

aidenn0 15 years ago |

I still remember one of my favorite linux bugs was due to the NULL behavior.

It was roughly this

read-from-p;

if(p != NULL) write-to-p;

since read-from-p is undefined if p is null, gcc could (and did) legally optimize out the NULL check, so you could end up writing to NULL.

[edit] I noticed that this case of bug is actually mentioned in the regehr article that is linked.

Peaker 15 years ago | |

I had a similar bug. Using a tracing framework of macros, I had something like:

    #define TRACE_FOO(foo_ptr)  TRACE_INT((foo_ptr)->x)

    ...

    TRACE_ENTER(TRACE_INT(x) TRACE_FOO(foo_ptr));
    if(!foo_ptr) return NULL;

    use foo-ptr

The NULL check was optimized out. The dereference of foo_ptr was hidden behind TRACE_FOO, which made it even harder to spot. I spent hours on that one :-)

ramidarigaz 15 years ago | |

If the compiler can optimize that away, what's the proper method for checking p?

panic 15 years ago | | |

Check p before using it. The issue is when you have code like the following, where the behavior of the first statement is only defined when the condition in the second is false:

    int x = *p;
    if (p == NULL) {
      // p can't be NULL without having triggered
      // undefined behavior in the first line, so
      // this code is removed by the compiler.
      return;
    }
    // ...

The fix is to stop dereferencing p before you know whether it's NULL or not:

    if (p == NULL) {
      // Moving the check before the dereference
      // avoids undefined behavior and resolves
      // the issue.
      return;
    }
    int x = *p;
    // ...

jergosh 15 years ago |

"You don't know. The computer may levitate." is what they used to say in my C course at the uni.

1amzave 15 years ago | |

I've heard "nasal demons" in some C circles (as in, "the compiler may cause demons to fly out your nose").

caf 15 years ago |

It is worth noting that unsigned overflow is guaranteed to be defined as 2's complement (wrapping) overflow, at least by Clang and other commonly available C compilers.

This is actually guaranteed by the standard (although the language used is not "2s complement", but "repeatedly adding or subtracting one more than the maximum value that can be stored in the type until the value is in range").

C requires that these sorts of type conversions happen through unions: using pointer casts is not correct and undefined behavior results.

Actually, loading a member of a union other than the one most recently stored to is undefined behaviour too - it's just that using a union in this way falls into the "...both Clang and GCC nail down a few behaviors that the C standard leaves undefined." category. (And most other compilers besides - there is much historical usage of this).

brimpa 15 years ago |

It would be nice to have a code snippet for each of the examples. I'm a fairly experienced C++ developer but my knowledge of compilers is, admittedly, lacking and I just want to make sure I'm on the same page.

Otherwise, a great read.

PS– Is it just me or is LLVM coming up more and more these days?

pixdamix 15 years ago | |

There's this patch from the linux-kernel:

- http://lkml.org/lkml/2009/7/17/187

- http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6...

I've put a gist file demonstrating the problem: https://gist.github.com/968723

  #include <stdio.h>
  
  struct agnx_priv {
      char demo;
  };
  
  struct ieee80211_hw {
      struct agnx_priv *priv;
  };
  
  struct pci_dev {
      struct ieee80211_hw* dev;
  };
  
  struct ieee80211_hw* pci_get_drvdata(struct pci_dev *pdev) {
  
      if(!pdev)
          return NULL;
  
      return pdev->dev;
  }
  
  static void agnx_pci_remove (struct pci_dev *pdev)
  {
      struct ieee80211_hw *dev = pci_get_drvdata(pdev);
      struct agnx_priv *priv = dev->priv;
  
      if (!dev) return;
  
      // This will segfault if the previous if is optimized
      printf("%c\n", priv->demo);
  }
  
  int main(int argc, char **argv)
  {
      // (struct pci_dev *)argv[1] is just to avoid compiler optimisations
      // =~ NULL if no args are passed.
      struct pci_dev * pdev = (struct pci_dev *)argv[1];
  
      agnx_pci_remove(pdev);
      return 0;
  }

Compile with:

  gcc -O2 -ggdb -Wall -Wundef -fno-strict-aliasing \
    -fno-common -fdelete-null-pointer-checks test-deref.c \
    -o test-deref

and

  gcc -O2 -ggdb -Wall -Wundef -fno-strict-aliasing \
    -fno-common -fno-delete-null-pointer-checks test-deref.c  \
    -o test-deref

olov 15 years ago |

Great article, shame it got optimized away before I had the chance to re-read it.

edit: seems Blogger turned their -O knob to eleven http://status.blogger.com/

KonradKlause 15 years ago |

Why does LLVM generate "ud2" instructions? WTF?

pascal_cuoq 15 years ago | |

Because it can. This is undefined behavior we are talking about.

KonradKlause 15 years ago | | |

Sorry, I don't get it. The Intel manual says on UD2:

"Raises an invalid opcode exception in all operating modes."

What is here undefined? LLVM must not generate such instructions except it it really wants such a exception. (Like Linux's panic() does on x86)

int x = *p; if (p == NULL) { // p can't be NULL without having triggered // undefined behavior in the first line, so // this code is removed by the compiler. return; } // ...

#include <stdio.h> struct agnx_priv { char demo; }; struct ieee80211_hw { struct agnx_priv *priv; }; struct pci_dev { struct ieee80211_hw* dev; }; struct ieee80211_hw* pci_get_drvdata(struct pci_dev *pdev) { if(!pdev) return NULL; return pdev->dev; } static void agnx_pci_remove (struct pci_dev *pdev) { struct ieee80211_hw *dev = pci_get_drvdata(pdev); struct agnx_priv *priv = dev->priv; if (!dev) return; // This will segfault if the previous if is optimized printf("%c\n", priv->demo); } int main(int argc, char **argv) { // (struct pci_dev *)argv[1] is just to avoid compiler optimisations // =~ NULL if no args are passed. struct pci_dev * pdev = (struct pci_dev *)argv[1]; agnx_pci_remove(pdev); return 0; }