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Re: Undetected fatal errors from redirected print


From: Andrew J. Schorr
Subject: Re: Undetected fatal errors from redirected print
Date: Mon, 22 Nov 2021 22:39:14 -0500
User-agent: Mutt/1.5.21 (2010-09-15)

Hi,

A few thoughts about this:

1. If the file open fails, you get a fatal error. So for example, if you don't 
have
permission to open the output file, it fails:

bash-4.2$ gawk 'BEGIN {print "hello" > "/dev/not.allowed"}'
gawk: cmd. line:1: fatal: cannot redirect to `/dev/not.allowed': Permission 
denied

2. In your case, the file open succeeded, but a subsequent write failed,
which is pretty unusual unless a filesystem has filled up.

If you look in io.c:close_redir, you'll see this:

...
        status = close_rp(rp, how);

        if (status != 0) {
                int save_errno = errno;
                char *s = strerror(save_errno);

                /*
                 * BWK's awk, as far back as SVR4 (1989) would check
                 * and warn about the status of close.  However, when
                 * we did this we got too many complaints, so we moved
                 * it to be under lint control.
                 */
                if (do_lint) {
                        if ((rp->flag & RED_PIPE) != 0)
                                lintwarn(_("failure status (%d) on pipe close 
of `%s': %s"),
                                         status, rp->value, s);
                        else if ((rp->flag & RED_TWOWAY) != 0)
                                lintwarn(_("failure status (%d) on two-way pipe 
close of `%s': %s"),
                                         status, rp->value, s);
                        else
                                lintwarn(_("failure status (%d) on file close 
of `%s': %s"),
                                         status, rp->value, s);
                }

                if (! do_traditional) {
                        /* set ERRNO too so that program can get at it */
                        update_ERRNO_int(save_errno);
                }
        }

So please try running again with --lint. I tested one case:

bash-4.2$ strace -fo trace.txt -P answer.txt -e fault=write gawk --lint 'BEGIN 
{print "42" > "answer.txt"; print close("answer.txt")}'
strace: Requested path 'answer.txt' resolved into '/tmp/answer.txt'
gawk: cmd. line:1: warning: failure status (-1) on file close of `answer.txt': 
Function not implemented
-1

Which leads to:

3. If you really care about whether the I/O operations succeeded, you should 
check
the return code from the close() function call, which is -1, indicating an error
occurred.

Regards,
Andy

On Mon, Nov 22, 2021 at 08:56:30PM -0500, Miguel Pineiro Jr. wrote:
> While surveying the io error handling of a few awks, I found that gawk 
> sometimes reports success upon a fatal failure to write a redirected print 
> (or printf) statement's data.
> 
> Fedora 35 Worstation
> Linux 5.14.17-301.fc35.x86_64
> GNU Awk 5.1.0, API: 3.0 (GNU MPFR 4.1.0-p13, GNU MP 6.2.0)
> GNU Awk, commit a08cc63b, 2021-09-09, the final version in master using 
> Autoconf 2.69.
> 
> I tested those two versions. I'm assuming the issue still exists in master, 
> but I did not confirm it. I did take a peek at `git diff a08cc63b..master 
> interpret.h builtin.c io.c` and none of the few changes appear to be a remedy 
> for the misbehavior I'm seeing.
> 
> My test script (which is at the end of this mail) tests the following 
> commands:
> 
> gawk 'BEGIN {print "42" > "answer.txt"}'
> gawk 'BEGIN {print "42" > "answer.txt"; close("answer.txt")}'
> gawk 'BEGIN {print "42"}' > answer.txt
> 
> It uses strace to intercept and sabotage attempts to write the number 42 into 
> answer.txt. If all were well, gawk would emit an error message and return a 
> non-zero exit status.
> 
> The first two commands have a redirected print statement. The first command 
> leaves the closing of the stream to gawk. The second, closes the stream 
> explicitly. In both cases, gawk fails to detect fatal io errors and returns 
> success.
> 
> GAWK CMD: gawk 'BEGIN {print "42" > "answer.txt"}'
> GAWK RET: 0
> BYTES WRITTEN: 0
> SYSCALL TRACE:  write(3, "42\n", 3)              = -1 ENOSYS (Function not 
> implemented) (INJECTED)
> 
> GAWK CMD: gawk 'BEGIN {print "42" > "answer.txt"; close("answer.txt")}'
> GAWK RET: 0
> BYTES WRITTEN: 0
> SYSCALL TRACE:  write(3, "42\n", 3)              = -1 ENOSYS (Function not 
> implemented) (INJECTED)
> 
> I didn't debug, but I did look at the code for a bit. I think the problem is 
> in close_rp. When it calls fclose/pclose on a redirected output stream with 
> pending data in its buffer, it fails to discriminate between a fatal write(2) 
> error during the flush and a subsequent, non-fatal close(2) error when 
> freeing the underlying file descriptor. The fatal write errors then slip 
> through as benign close errors.
> 
> Un-redirected output is not affected. On its way out, in close_io, gawk 
> flushes stdout and stderr. If either flush fails, the failure is detected, a 
> warning produced, and a non-zero status returned. All good. 
> 
> GAWK CMD: gawk 'BEGIN {print "42"}' > answer.txt
> gawk: warning: error writing standard output: Function not implemented
> GAWK RET: 1
> BYTES WRITTEN: 0
> SYSCALL TRACE:  write(1, "42\n", 3)              = -1 ENOSYS (Function not 
> implemented) (INJECTED)
> 
> In case it's of use to anyone, my script follows. It requires strace and it 
> will create two files in the working directory, answer.txt and trace.txt.
> 
> Take care,
> Miguel
> 
> 
> # Commands to be tested served on stdin
> exec <<'EOF'
> gawk 'BEGIN {print "42" > "answer.txt"}'
> gawk 'BEGIN {print "42" > "answer.txt"; close("answer.txt")}'
> gawk 'BEGIN {print "42"}' > answer.txt
> EOF
> 
> # Sabotage gawk's attempt to write "42" to answer.txt.
> while IFS= read -r gcmd; do
>       echo
>       >answer.txt >trace.txt
>       echo GAWK CMD: "$gcmd"
>       strace -fo trace.txt -P answer.txt -e fault=write sh -c "$gcmd"
>       echo GAWK RET: $?
>       echo BYTES WRITTEN: $(wc -c < answer.txt)
>       echo SYSCALL TRACE: "$(sed -n '/write/s/^[^ ]*//p' trace.txt)"
> done



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