I was recently debugging a strange crash in a downstream project which turned
out to be a QEMU bug related to the `mremap` implementation in linux-user. In
practice, this bug essentially led to arbitrary memory regions being unmapped
when a 32-bit guest, running on a 64-bit host, uses `mremap` to shrink a memory
mapping.

The first patch in this set resolves that bug. Since the patch is very simple,
and the bug is quite likely to be hit, I suspect that that commit is a good
candidate for qemu-stable.

The following two patches just resolve two more bugs I became aware of whilst
working on this code. I believe the messages in those patches contain all
necessary context. They are less critical and the fixes more complex, so are
likely not suitable for backporting into qemu-stable.

The final commits adds tcg tests for the fixed `mremap` behavior. The third fix
is unfortunately difficult to test programmatically, but I have confirmed that
it behaves as expected by observing the output of `strace qemu-i386 repro`,
where `repro` is the following C program:

    #define _GNU_SOURCE
    #include <stddef.h>
    #include <sys/mman.h>
    int main(void) {
        char *a = mmap(NULL, 4097, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        char *b = mmap(NULL, 4097, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        mremap(b, 4097, 4097, MREMAP_FIXED | MREMAP_MAYMOVE, a);
        // QEMU has now leaked a page of its memory reservation!
        return 0;
    }

Prior to the patch, as the comment says, QEMU leaks a page of its address space
reservation (i.e. the page becomes unmapped). After the patch, QEMU correctly
reclaims that page with `mmap`.

Matthew Lugg (4):
  linux-user: fix mremap unmapping adjacent region
  linux-user: fix mremap errors for invalid ranges
  linux-user: fix reserved_va page leak in do_munmap
  tests: add tcg coverage for fixed mremap bugs

 linux-user/mmap.c               | 75 +++++++++++++--------------------
 tests/tcg/multiarch/test-mmap.c | 47 ++++++++++++++++++---
 2 files changed, 71 insertions(+), 51 deletions(-)

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2.51.0