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dev: raw disks are 64-bit, so a real drive stops wrapping at 4 GB

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struct file already carried a uint64 offset and bread already took a
uint64 block number. The narrowing was one boundary in the middle:
devsw's read and write took a 32-bit uint, so fileread passed a 64-bit
f->off into a 32-bit parameter and the top half was discarded.

The effect was not a refusal at 4 GB but silent wraparound past it. A
read at 5 GB returned the bytes at 1 GB, reporting success. disk.c had a
DISK_MAX_BYTES constant documenting the limit, and nothing anywhere
referenced it -- the cap was described but never enforced, which is the
worst of the three options.

The signature is now uint64 across all nine files that carry it, and
diskread/diskwrite no longer narrow the block number on the way to
bread. Widening this is a prerequisite for NVMe and for any real disk:
namespaces start well above 4 GB and the failure mode reads valid data
from the wrong place.

Verified end to end rather than by inspection. A 6 GB sparse image with
"PAST4GB-MARKER-OK" written at byte 5368709120, attached as /dev/hdb to a
ramdisk-root kernel so device 1 was free:

  # dd if=/dev/hdb bs=4096 skip=1310720 count=1 | od -c | head -2
  0000000   P   A   S   T   4   G   B   -   M   A   R   K   E   R   -   O

Under the old code that offset truncates to 1 GB, where the image is
zeroes. The whole path is exercised: f->off, devsw, diskread, bread and
ide.c's LBA48.

Worth recording for next time: `make ramroot` stages the ESP itself, and
running `make esp` afterwards rebuilds the kernel without RAMROOT and
silently undoes it. The tell is a 855 KB kernel where a 25 MB one is
expected.

110 [ok], no failures, on the default build.
This commit is contained in:
Nexoniarz 2026-08-11 12:15:38 +02:00
commit ad2b321d56
9 changed files with 42 additions and 41 deletions
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4
include/file.h
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@ -123,8 +123,8 @@ struct inode {
// table mapping major device number to
// device functions
struct devsw {
int (*read)(struct inode*, uint, char*, int);
int (*write)(struct inode*, uint, char*, int);
int (*read)(struct inode*, uint64, char*, int);
int (*write)(struct inode*, uint64, char*, int);
// Optional. Returns 0 when a read would block, which is the only thing
// fileread needs to know to honour O_NONBLOCK without every device driver
// growing a flags argument. A device that omits this is treated as always