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Subject: Re: pread()/pwrite() fail with EBADF in child process if already used
 before fork()
To: cygwin AT cygwin DOT com
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Date: Mon, 23 Sep 2024 14:52:52 +0200
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From: Christian Franke via Cygwin <cygwin AT cygwin DOT com>
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Christian Franke via Cygwin wrote:
> Found during test of 'stress-ng --pseek ...' from current upstream 
> stress-ng git HEAD:
>
> Testcase:
>
> $ uname -r
> 3.5.4-1.x86_64
>
> $ cat pfail.c
> #include <fcntl.h>
> #include <stdio.h>
> #include <unistd.h>
> #include <sys/wait.h>
>
> int main()
> {
>   int fd = open("pwrite.tmp", O_RDWR|O_CREAT|O_BINARY, 0666);
>   if (fd < 0) {
>     perror("open"); return 1;
>   }
>
>   char c = 42;
>   if (pwrite(fd, &c, 1, 0) < 0)
>     perror("pwrite");
>
>   if (fork() == 0) {
>     if (pread(fd, &c, 1, 0) < 0)
>       perror("pread");
>     _exit(0);
>   }
>
>   int status;
>   wait(&status);
>   return 0;
> }
>
> $ make pfail
> cc     pfail.c   -o pfail
>
> $ ./pfail
> pread: Bad file descriptor
>
> $ strace ./pfail
> ...
>   617   75356 [main] pfail 10826 dofork: 10827 = fork()
>    82   11289 [main] pfail 10827 seterrno_from_nt_status: 
> /usr/src/debug/cygwin-3.5.4-1/winsup/cygwin/fhandler/disk_file.cc:1883 
> status 0xC0000008 -> windows error 6
>    80   75436 [main] pfail 10826 wait4: calling proc_subproc, pid -1, 
> options 0
>    76   11365 [main] pfail 10827 geterrno_from_win_error: windows 
> error 6 == errno 9
>    78   75514 [main] pfail 10826 proc_subproc: args: 5, -7728
>    64   11429 [main] pfail 10827 pread: -1 = pread(3, 0x7FFFFCC0B, 1, 
> 0), errno 9
> ...
>
>
> The problem does not occur if there is no pread()/pwrite() before the 
> fork(). This suggests that the child process inherits the extra handle 
> value used to keep the original seek position, but not the actual handle.
>


The mentioned extra handle 'prw_handle' is set to null after fork here:
https://cygwin.com/git/?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/fhandler/disk_file.cc;h=f4c21d3#l1698

But a test suggests that fhandler_disk_file::fixup_after_fork() is never 
called or debug_printf() does not work if called in this function.

Thoughts?


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