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Fix fork() crash on Windows

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On Windows, sometimes fork() could crash with message likes:

    fork() ViewOrDie(170000) failed with win32 error 487

This is due to a bug in our file descriptor inheritance. We have cursors
which are shared between processes. They let us track the file positions
of read() and write() operations. At startup they were being mmap()ed to
memory addresses that were assigned by WIN32. That's bad because Windows
likes to give us memory addresses beneath the program image in the first
4mb range that are likely to conflict with other assignments. That ended
up causing problems because fork() needs to be able to assume that a map
will be possible to resurrect at the same address. But for one reason or
another, Windows libraries we don't control could sneak allocations into
the memory space that overlap with these mappings. This change solves it
by choosing a random memory address instead when mapping cursor objects.
This commit is contained in:
Justine Tunney 2024-10-12 15:26:32 -07:00
parent 5edc0819c0
commit dc1afc968b
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GPG key ID: BE714B4575D6E328
4 changed files with 74 additions and 4 deletions
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4
libc/proc/fork-nt.c
View file

@ -125,11 +125,9 @@ static dontinline textwindows ssize_t ForkIo2(
static dontinline textwindows bool WriteAll(int64_t h, void *buf, size_t n) {
bool ok;
ok = ForkIo2(h, buf, n, (void *)WriteFile, "WriteFile", false) != -1;
if (!ok) {
if (!ok)
STRACE("fork() failed in parent due to WriteAll(%ld, %p, %'zu) → %u", h,
buf, n, GetLastError());
__print_maps(0);
}
return ok;
}