[Bug 248223] Linuxulator: pread behavior differences

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=248223

            Bug ID: 248223
           Summary: Linuxulator: pread behavior differences
           Product: Base System
           Version: 12.1-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: iwtcex at gmail.com

% cat pread.c
#define _GNU_SOURCE

#include <assert.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>

void pread_v(int fd, void* buf, size_t nbytes, off64_t offset) {
  printf("pread(_, _, 0x%lx, _) -> ", nbytes);
  int ret = pread(fd, buf, nbytes, offset);
  if (ret != -1) {
    printf("0x%x\n", ret);
  } else {
    printf("%s\n", strerror(errno));
  }
}

int main() {

  int mem = open("/proc/self/mem", O_RDONLY);
  assert(mem != -1);

  char buf[0x4000];

  pread_v(mem, buf, 0x1000, 0); // should be EIO
  printf("---\n");

  void* p1 = mmap(NULL,        0x2000, PROT_READ | PROT_WRITE, MAP_ANON |
MAP_PRIVATE,             -1, 0);
  void* p2 = mmap(p1 + 0x1000, 0x1000, PROT_NONE,              MAP_ANON |
MAP_PRIVATE | MAP_FIXED, -1, 0);

  assert(p1 != MAP_FAILED);
  assert(p2 == p1 + 0x1000);

  assert(munmap(p2, 0x1000) == 0);

  for (int n = sizeof(buf); n >= 0x1000; n /= 2) {
    pread_v(mem, buf, n, (off64_t)p1); // should return 0x1000
  }

  return 0;
}
% /compat/linux/bin/gcc -std=c99 -Wall pread.c -o pread
% ./pread
pread(_, _, 0x1000, _) -> Bad address
---
pread(_, _, 0x4000, _) -> Bad address
pread(_, _, 0x2000, _) -> Bad address
pread(_, _, 0x1000, _) -> 0x1000

On the other hand, Linux (Xubuntu 18.04.3) prints Input/Output errors and
0x1000, 0x1000, 0x1000 there.

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