On Wed, 21 Feb 2024 12:48:34 -0800 Jason Thorpe <thor...@me.com> wrote:
> > > On Feb 21, 2024, at 2:52 AM, Sad Clouds <cryintotheblue...@gmail.com> wrote: > > > > Hello, for most operating systems determining the size of a block > > device can be done with: > > > > lseek(fd, 0, SEEK_END); > > On what operating systems does this do what you claim? > > > However, on NetBSD this does not seem to work. > > It doesn’t work on macOS, either: > > thorpej-mbp:thorpej 541$ sudo ./lseek /dev/disk4 > Password: > lseek(fd, 4096, SEEK_SET) = 4096 bytes > lseek(fd, 0, SEEK_END) = 0 bytes > thorpej-mbp:thorpej 542$ > > -- thorpej > Hello, it works on Linux, FreeBSD, Solaris and it also works on NetBSD but only with ld(4) driver, with SCSI or SATA drives, lseek returns bogus size of 0 bytes, even though the kernel knows the size of block device, otherwise partition formatting would be impossible. Specifically for NetBSD: # uname -mps NetBSD evbarm earmv7hf # dmesg | grep ld0 [ 1.768438] ld0 at sdmmc0: <0x03:0x5344:SE16G:0x80:0x08834ec0:0x0d8> [ 1.768438] ld0: 15193 MB, 7717 cyl, 64 head, 63 sec, 512 bytes/sect x 31116288 sectors [ 1.798199] ld0: 4-bit width, High-Speed/SDR25, 50.000 MHz [ 2.338199] boot device: ld0 [ 2.338199] root on ld0a dumps on ld0b Initially stat on /dev/ld0 reports size as 0 bytes: # stat -x /dev/ld0 File: "/dev/ld0" Size: 0 Blocks: 0 IO Block: 2048 Block Device Device: 92,0 Inode: 1573 Links: 1 Mode: (0640/brw-r-----) Uid: ( 0/ root) Gid: ( 5/operator) Access: 2024-02-21 13:30:44.666619198 +0000 Modify: 2021-10-17 17:58:03.000000000 +0100 Change: 2021-10-17 17:58:03.000000000 +0100 Birth: 1970-01-01 01:00:00.000000000 +0100 So stat is no good here, let's try lseek: # gcc -Wall -Wpedantic lseek_test.c && ./a.out lseek(fd, 4096, SEEK_SET) = 4096 bytes lseek(fd, 0, SEEK_END) = 15931539456 bytes But now, after lseek, stat reports the correct size: # stat -x /dev/ld0 File: "/dev/ld0" Size: 15931539456 Blocks: 0 IO Block: 2048 Block Device Device: 92,0 Inode: 1573 Links: 1 Mode: (0640/brw-r-----) Uid: ( 0/ root) Gid: ( 5/operator) Access: 2024-02-21 13:30:44.666619198 +0000 Modify: 2021-10-17 17:58:03.000000000 +0100 Change: 2021-10-17 17:58:03.000000000 +0100 Birth: 1970-01-01 01:00:00.000000000 +0100 This is such a buggy behaviour that I stopped using stat for finding out block device size a long time ago, and not just on NetBSD.
