[Qemu-devel] [Bug 1810433] Re: aarch64-linux-user master: inconsistent pwrite behaviour

[Peter Maydell]

Proposed patch at https://patchwork.ozlabs.org/patch/1022092/

NB: my guess is that your pwrite on 32-bit arm test is behaving like
that because it isn't going via the pwrite64 syscall, or possibly
because glibc there is dealing with the NULL special case early. Use
QEMU's -strace argument (or strace on real h/w) to see what libc is
actually turning that pwrite() function call into at the syscall level.


** Changed in: qemu
       Status: New => In Progress

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Title:
  aarch64-linux-user master: inconsistent pwrite behaviour

Status in QEMU:
  In Progress

Bug description:
  Hello,

  I am running aarch64-linux-user from master, commit
  20d6c7312f1b812bb9c750f4087f69ac8485cc90

  And I've found the following inconsistent emulation of pwrite() call when 
buf==NULL and len=0.
  Minimal reproducible sample is the following:

  #define _GNU_SOURCE
  #include <stdlib.h>
  #include <stdio.h>
  #include <unistd.h>
  #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  #include <string.h>

  /*
   System                  | Result
  -------------------------+----------------
   Native x86_64 4.12.14   | pwrite ret = 0
   Native aarch64 4.4.159  | pwrite ret = 0
   qemu-aarch64 at x86_64  | pwrite ret = -1
     ( 20d6c7312f1b8 )     |
  */

  int main(int argc, char** argv) {
   int fd = open("test.dat", O_CREAT | O_RDWR, 0644);
   if (fd < 0) {
    perror("open");
    return 1;
   }

   int ret = fallocate(fd, 0, 0, 1000);
   if (ret < 0) {
    perror("fallocate");
    return 1;
   }

   ssize_t ret_pwrite = pwrite(fd, NULL, 0, 0);
   printf("pwrite ret = %ld\n", ret_pwrite);

   close(fd);

   return 0;
  }

  
  Please note, that the same binary executable prints different output at 
native aarch64 platform and under aarch64-linux-user

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