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

[Peter Maydell]

This seems related to commit 58cfa6c2e6eb51b23cc98f8, where this was
fixed for the write() syscall. In that commit message the author writes
"Q. Do pread64/pwrite64 need to be changed similarly? A. Experiment
suggests not: both linux and linux-user yield -1 for NULL 0-length
reads/writes." That doesn't match your results, though, and looking at
the source both write and pwrite syscalls go through the vfs_write()
function, so their behaviour for a NULL/0 buffer should be identical.

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https://bugs.launchpad.net/bugs/1810433

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

Status in QEMU:
  New

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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