Package: coreutils Version: 8.32-4+b1 Severity: normal Dear Maintainer,
Consider the following: $ printf 'abcdAA' > file $ printf '_D' | dd of=file skip=1 iflag=skip_bytes seek=3 oflag=seek_bytes status=none What do you expect file to contain? I, very obviously, expect to: 1. seek to position 3 2. truncate there 3. skip 1 byte 4. read the rest => EOF => write it for "abcD". But it's not: it's 00 00 00 44, and strace reveals that file was opened with O_TRUNC. If we repeat the same exercise but spec bs=1 (or obs=1, these are equivalent for the purposes of this report), we get the expected value (and see ftruncate(1, 3)). The same happens for bs=2 and bs=3! bs=4 yields the original "\0\0\0D" again, though. I had expected this to be an overzealous and backward interpretation of the standard, which reads > If seek= expr is specified, but conv= notrunc is not, the effect of the > copy shall be to preserve the blocks in the output file over which dd > seeks, but no other portion of the output file shall be preserved. but that's not the case, and coreutils appears to have extended this logically as expected when faced with sub-block seek offsets (the standard is specced entirely in terms of blocks, but if you spec seek_bytes the intent is to obviously preserve everything prior to the seek offset, rather than just the sought-over blocks ‒ this is demonstrated by bs=2 being fine, the counterfactual would be "ab\0D"). This makes me believe this is a simple programming error when optimising for "pick O_TRUNC instead of ftruncate(1, 0) later", like in https://git.sr.ht/~nabijaczleweli/voreutils/tree/7f7fbb1d95b66e832e339324c298ff5c8721a570/item/cmd/dd.cpp#L373-378 except that it probably checks for !(seek_off / obs) which is, obviously, 0 for 3/≥4. Best, наб -- System Information: Debian Release: 11.4 APT prefers stable-updates APT policy: (500, 'stable-updates'), (500, 'stable-security'), (500, 'stable-debug'), (500, 'stable') Architecture: amd64 (x86_64) Foreign Architectures: i386 Kernel: Linux 5.10.0-15-amd64 (SMP w/24 CPU threads) Kernel taint flags: TAINT_PROPRIETARY_MODULE, TAINT_FIRMWARE_WORKAROUND, TAINT_OOT_MODULE, TAINT_UNSIGNED_MODULE Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8), LANGUAGE=en_GB:en Shell: /bin/sh linked to /usr/bin/dash Init: systemd (via /run/systemd/system) LSM: AppArmor: enabled Versions of packages coreutils depends on: ii libacl1 2.2.53-10 ii libattr1 1:2.4.48-6 ii libc6 2.31-13+deb11u3 ii libgmp10 2:6.2.1+dfsg-1+deb11u1 ii libselinux1 3.1-3 coreutils recommends no packages. coreutils suggests no packages. -- no debconf information
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