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src: remap invalid file descriptors using dup2
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When checking for the validity of the stdio file descriptors (nodejs#875),
ones which don't exist are intended to be remapped to /dev/null (and, if
that doesn't work, we abort).

This however doesn't work on all platforms and in all cases (e.g. /dev/null
could already have been opened by the acting process and not actually be
mapped to the expected file descriptor); instead, use the `dup2` syscall
as a more robust solution (conforms to POSIX.1).
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obiwac committed Aug 30, 2022
1 parent 63aba56 commit 7ad5f30
Showing 1 changed file with 16 additions and 4 deletions.
20 changes: 16 additions & 4 deletions src/node.cc
Original file line number Diff line number Diff line change
Expand Up @@ -607,12 +607,24 @@ static void PlatformInit(ProcessInitializationFlags::Flags flags) {
// anything.
for (auto& s : stdio) {
const int fd = &s - stdio;
if (fstat(fd, &s.stat) == 0) continue;
if (fstat(fd, &s.stat) == 0)
continue;

// Anything but EBADF means something is seriously wrong. We don't
// have to special-case EINTR, fstat() is not interruptible.
if (errno != EBADF) ABORT();
if (fd != open("/dev/null", O_RDWR)) ABORT();
if (fstat(fd, &s.stat) != 0) ABORT();
if (errno != EBADF)
ABORT();

// If EBADF (file descriptor doesn't exist), open /dev/null and duplicate
// its file descriptor to the invalid file descriptor. Make sure *that*
// file descriptor is valid.
int null_fd = open("/dev/null", O_RDWR);
if (dup2(null_fd, fd) != 0)
ABORT();

if (fstat(fd, &s.stat) == 0)
continue;
ABORT();
}
}

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