dd: writes null bytes to device if stdout is redirect to device

[jhscheer]

@jfinkels I took the liberty to open a new issue with a summary for all the findings you published in #3447

gnu/tests/dd/skip-seek-past-dev.sh runs

device=$(df --output=source . | tail -n1)
dev_size=$(blockdev --getsize64 $device)
DEV_OFLOW=$(expr $dev_size + 1)
dd bs=1 seek=$DEV_OFLOW count=0 status=noxfer > "$device"

and expects $?=1 and stderr=dd: 'standard output': cannot seek: Invalid argument.

However, uu_tail does not check if stdout is a redirect to a seekable device and then writes null bytes to $device !

coreutils/src/uu/dd/src/dd.rs

Lines 301 to 309 in d894847

	let mut dst = io::stdout();
	// The --seek and --oseek flags are additive.
	let amt = seek.unwrap_or(0) + oseek.unwrap_or(0);
	// stdout is not seekable, so we just write null bytes.
	if amt > 0 {
	io::copy(&mut io::repeat(0u8).take(amt as u64), &mut dst)
	.map_err_context(|| String::from("write error"))?;
	}

possible fix

Haven't looked into it much but a possible fix could be to get the redirect for /dev/stdout similar to this:

coreutils/src/uu/stat/src/stat.rs

Lines 558 to 562 in d894847

	if let Ok(p) = Path::new("/dev/stdin").canonicalize() {
	p.into_os_string()
	} else {
	OsString::from("/dev/stdin")
	}

and then apply the seek checks for the new path.

[jfinkels]

Thank you for creating a new issue 👍