On 17/05/2021 23:00, John Nielsen wrote:
Hi all-
I’m not much of a kernel programmer but I’m trying to maintain/improve the
isboot module, which allows booting directly from iSCSI by reading the iSCSI
Boot Firmware Table (iBFT), bringing up the interface with the details
specified therein and connecting to the specified iSCSI target before trying
to mount root.
I’m not the original author but as the port maintainer I am hosting the code
here: https://github.com/jnielsendotnet/isboot
I have a test system where the module loads but fails to find the iBFT. I
reviewed the iBFT code and realized it has a bunch of magic numbers mixed in
with some random memory diving. If I’m reading it right (see
https://github.com/jnielsendotnet/isboot/blob/master/src/ibft.h#L37 and
https://github.com/jnielsendotnet/isboot/blob/master/src/ibft.c#L521), it
looks like it scans all of the (kernel?) memory between 512K and 1M in
16-byte increments looking for one beginning with the string “iBFT”, which if
it finds will be used as the offset for reading the table. I don’t know where
the 512K and 1M values came from or if they are correct, but I do have a
system where that method does not work.
They seem to come from iBFT specification.
Locating the iBFT
The iBFT can be located by the Low RAM Method. Scan for the table header
signature in system memory between 512K and 1024K. The scan MUST be done
starting at the lower address scanning forward to the higher address. When
using the Low RAM Method, the table header must be aligned on a 16-byte
boundary.
IIUC, the iBFT is an ACPI table, and it seems like using ACPI to find it
would be safer and more reliable. So my question is: how does one do that?
Are there other places in the kernel code that do this sort of thing that I
could use as a model?
Yes.
Search for occurrences of AcpiGetTable() in the code, that's the function you
want to use.
But it could be that the table gets overwritten during boot for some reason...
--
Andriy Gapon