macOS getxattr(2) is broken on SMB shares

getxattr.c

/*
  macOS getxattr(2) is broken on SMB shares:

    * Cannot read xattr larger than 1048576 bytes.
    * Fails by ERANGE (Result too large) if 'size' argument is smaller than
      entire xattr size.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/errno.h>
#include <sys/xattr.h>

int main(int argc, char **argv) {
  setbuf(stdout, NULL);

  if (argc < 3) {
    printf("Usage: %s PATH NAME [READ_CHUNK_SIZE]\n", argv[0]);
    exit(1);
  }

  const char *path = argv[1];
  const char *ea_name = argv[2];
  size_t ea_read_chunk_size_max = (argc >= 4) ? strtol(argv[3], NULL, 10) : 0;
  printf("path:         %s\n", path);
  printf("ea name:      %s\n", ea_name);

  ssize_t ea_size = getxattr(path, ea_name, NULL, 0, 0, 0);
  if (ea_size == -1) {
    fprintf(stderr, "%s: ERROR: getxattr failed: %s: %s: %s\n",
      argv[0], path, ea_name, strerror(errno));
    exit(1);
  }
  printf("ea size:      %ld\n", (long)ea_size);

  unsigned char *ea_value = malloc(ea_size);
  if (ea_value == NULL) {
    fprintf(stderr, "%s: ERROR: malloc failed: %s\n",
      argv[0], strerror(errno));
    exit(1);
  } 

  u_int32_t ea_read_offset = 0;
  unsigned char *ea_value_ptr = ea_value;
  size_t ea_read_remained = ea_size;
  ssize_t ea_read_chunk_size;
  for (long i=1; ea_read_offset < ea_size; i++) {
    if (ea_read_chunk_size_max == 0 || ea_read_chunk_size_max > ea_read_remained) {
      ea_read_chunk_size = ea_read_remained;
    } else {
      ea_read_chunk_size = ea_read_chunk_size_max;
    }
    ea_read_chunk_size = getxattr(path, ea_name, ea_value_ptr, ea_read_chunk_size, ea_read_offset, 0);
    if (ea_read_chunk_size == -1) {
      fprintf(stderr, "%s: ERROR: getxattr failed: %s: %s: %s\n",
	argv[0], path, ea_name, strerror(errno));
      exit(1);
    }
    printf("  loop %8ld: read chunk size=%ld, offset=%ld\n", i, (long)ea_read_chunk_size, (long)ea_read_offset);

    ea_value_ptr += ea_read_chunk_size;
    ea_read_offset += ea_read_chunk_size;
    ea_read_remained -= ea_read_chunk_size;
  }
  printf("ea read size: %ld\n", (long)ea_read_offset);
}

gistfile1.txt

$ ./getxattr '/Volumes/smb/foo.txt' com.apple.ResourceFork
path:         /Volumes/smb/foo.txt
ea name:      com.apple.ResourceFork
ea size:      6682679
  loop        1: read chunk size=1048576, offset=0
./getxattr: ERROR: getxattr failed: /Volumes/smb/foo.txt: com.apple.ResourceFork: Result too large

$ ./getxattr '/Volumes/smb/foo.txt' com.apple.ResourceFork 100000
path:         /Volumes/smb/foo.txt
ea name:      com.apple.ResourceFork
ea size:      6682679
./getxattr: ERROR: getxattr failed: /Volumes/smb/foo.txt: com.apple.ResourceFork: Result too large