[llvm] r219170 - Support: Add a utility to remap std{in, out, err} to /dev/null if closed

[David Majnemer]

fileno(stdout) will be 1 even if file descriptor 1 is closed.

On Mon, Oct 6, 2014 at 4:43 PM, Reid Kleckner <rnk at google.com> wrote:

> I thought we agreed to reduce our reliance on STDOUT_FILENO and instead
> check fileno(stdout)?
>
> On Mon, Oct 6, 2014 at 4:16 PM, David Majnemer <david.majnemer at gmail.com>
> wrote:
>
>> Author: majnemer
>> Date: Mon Oct  6 18:16:18 2014
>> New Revision: 219170
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=219170&view=rev
>> Log:
>> Support: Add a utility to remap std{in,out,err} to /dev/null if closed
>>
>> It's possible to start a program with one (or all) of the standard file
>> descriptors closed.  Subsequent open system calls will give the program
>> a low-numbered file descriptor.
>>
>> This is problematic because we may believe we are writing to standard
>> out instead of a file.
>>
>> Introduce Process::FixupStandardFileDescriptors, a helper function to
>> remap standard file descriptors to /dev/null if they were closed before
>> the program started.
>>
>> Modified:
>>     llvm/trunk/include/llvm/Support/Process.h
>>     llvm/trunk/lib/Support/Unix/Process.inc
>>     llvm/trunk/lib/Support/Windows/Process.inc
>>
>> Modified: llvm/trunk/include/llvm/Support/Process.h
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/Process.h?rev=219170&r1=219169&r2=219170&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/include/llvm/Support/Process.h (original)
>> +++ llvm/trunk/include/llvm/Support/Process.h Mon Oct  6 18:16:18 2014
>> @@ -186,6 +186,12 @@ public:
>>                      ArrayRef<const char *> ArgsFromMain,
>>                      SpecificBumpPtrAllocator<char> &ArgAllocator);
>>
>> +  // This functions ensures that the standard file descriptors (input,
>> output,
>> +  // and error) are properly mapped to a file descriptor before we use
>> any of
>> +  // them.  This should only be called by standalone programs, library
>> +  // components should not call this.
>> +  static std::error_code FixupStandardFileDescriptors();
>> +
>>    /// This function determines if the standard input is connected
>> directly
>>    /// to a user's input (keyboard probably), rather than coming from a
>> file
>>    /// or pipe.
>>
>> Modified: llvm/trunk/lib/Support/Unix/Process.inc
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/Unix/Process.inc?rev=219170&r1=219169&r2=219170&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/lib/Support/Unix/Process.inc (original)
>> +++ llvm/trunk/lib/Support/Unix/Process.inc Mon Oct  6 18:16:18 2014
>> @@ -18,6 +18,9 @@
>>  #include "llvm/Support/Mutex.h"
>>  #include "llvm/Support/MutexGuard.h"
>>  #include "llvm/Support/TimeValue.h"
>> +#if HAVE_FCNTL_H
>> +#include <fcntl.h>
>> +#endif
>>  #ifdef HAVE_SYS_TIME_H
>>  #include <sys/time.h>
>>  #endif
>> @@ -199,6 +202,62 @@ Process::GetArgumentVector(SmallVectorIm
>>    return std::error_code();
>>  }
>>
>> +namespace {
>> +class FDCloser {
>> +public:
>> +  FDCloser(int &FD) : FD(FD), KeepOpen(false) {}
>> +  void keepOpen() { KeepOpen = true; }
>> +  ~FDCloser() {
>> +    if (!KeepOpen && FD >= 0)
>> +      ::close(FD);
>> +  }
>> +
>> +private:
>> +  FDCloser(const FDCloser &) LLVM_DELETED_FUNCTION;
>> +  void operator=(const FDCloser &) LLVM_DELETED_FUNCTION;
>> +
>> +  int &FD;
>> +  bool KeepOpen;
>> +};
>> +}
>> +
>> +std::error_code Process::FixupStandardFileDescriptors() {
>> +  int NullFD = -1;
>> +  FDCloser FDC(NullFD);
>> +  const int StandardFDs[] = {STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO};
>> +  for (int StandardFD : StandardFDs) {
>> +    struct stat st;
>> +    errno = 0;
>> +    while (fstat(StandardFD, &st) < 0) {
>> +      assert(errno && "expected errno to be set if fstat failed!");
>> +      // fstat should return EBADF if the file descriptor is closed.
>> +      if (errno == EBADF)
>> +        break;
>> +      // retry fstat if we got EINTR, otherwise bubble up the failure.
>> +      if (errno != EINTR)
>> +        return std::error_code(errno, std::generic_category());
>> +    }
>> +    // if fstat succeeds, move on to the next FD.
>> +    if (!errno)
>> +      continue;
>> +    assert(errno == EBADF && "expected errno to have EBADF at this
>> point!");
>> +
>> +    if (NullFD < 0) {
>> +      while ((NullFD = open("/dev/null", O_RDWR)) < 0) {
>> +        if (errno == EINTR)
>> +          continue;
>> +        return std::error_code(errno, std::generic_category());
>> +      }
>> +    }
>> +
>> +    if (NullFD == StandardFD)
>> +      FDC.keepOpen();
>> +    else if (dup2(NullFD, StandardFD) < 0)
>> +      return std::error_code(errno, std::generic_category());
>> +  }
>> +  return std::error_code();
>> +}
>> +
>>  bool Process::StandardInIsUserInput() {
>>    return FileDescriptorIsDisplayed(STDIN_FILENO);
>>  }
>>
>> Modified: llvm/trunk/lib/Support/Windows/Process.inc
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/Windows/Process.inc?rev=219170&r1=219169&r2=219170&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/lib/Support/Windows/Process.inc (original)
>> +++ llvm/trunk/lib/Support/Windows/Process.inc Mon Oct  6 18:16:18 2014
>> @@ -273,6 +273,10 @@ Process::GetArgumentVector(SmallVectorIm
>>    return ec;
>>  }
>>
>> +std::error_code Process::FixupStandardFileDescriptors() {
>> +  return std::error_code();
>> +}
>> +
>>  bool Process::StandardInIsUserInput() {
>>    return FileDescriptorIsDisplayed(0);
>>  }
>>
>>
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>>
>
>
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