On 19/06/19 09:20, Szőts Ákos wrote: > Dear List members, > > I've recently discovered that od "--skip-bytes" feature doesn't > actually seek into the file but rather it reads everything from the > very beginning until the given offset is reached. > > This can be problematic in two cases: > - when speed matters, and > - when parts of the file is read-protected (like in the case of > /dev/mem when CONFIG_STRICT_DEVMEM=y, which is usually the case). In > this example od will reach an area in /dev/mem which is read-protected > (even for root) and will report back EPERM before it could reach the > requested, and allowed, memory area. The error report will be > misleading since the resulting EPERM is not because the requested area > is protected (as suggested by the error message) but because of the > inner mechanics of the program. > > In contrary, hexdump jumps to the address directly. > > You can test it by running the following two commands: > - strace hexdump --skip 0xFE000124 --length 1 /dev/sda > - strace od --skip-bytes 0xFE000124 --read-bytes 1 /dev/sda > > I don't know whether this behaviour is intended but if so, could you > please expand the documentation a little bit with this additional > information? > > od version: od (GNU coreutils) 8.30, openSUSE Tumbleweed, x64 > > Thank you and all the best,
Similar issues were discussed in https://debbugs.gnu.org/18621 (though od always only skipped in regular files). It's hard to get something applicable to all cases. We'd like to seek for the last two here: $ stat /proc/$$/io /sys/kernel/vmcoreinfo /dev/sda /dev/mem | grep Size Size: 0 Blocks: 0 IO Block: 1024 regular empty file Size: 4096 Blocks: 0 IO Block: 4096 regular file Size: 0 Blocks: 0 IO Block: 4096 block special file Size: 0 Blocks: 0 IO Block: 4096 character special file Maybe we should relax the cases we do read() for, and try to seek in block/character special files, falling back to read() where that fails? Note one can get the same functionality by combining dd like: dd bs=1 skip=$((0xFE000124)) count=1 if=/dev/sda | od or more efficiently for larger sizes like dd iflag=skip_bytes skip=$((0xFE000124)) if=/dev/sda | od --read-bytes=1 cheers, Pádraig
