[SeaBIOS] Problems with Linux payload

[Scott Duplichan]

Laszlo Ersek [mailto:lersek at redhat.com] wrote:

    [snip]

]Can you hex-dump the guest memory too, in both cases, starting from
]0xf4b78+0x18, and from 0x1ffec000+0x18, respectively, for 0x8 bytes?
]This would provide the segment descriptor in each case that the selector
]value 0x18 references.

I thought of the same, but then saw that gdb is actually dumping those:

    0018 000f0000 0000ffff 00009b00 DPL=0 CS16 (causes reset)
    0018 00000000 ffffffff 00c09300 DPL=0 DS   (works)

]In the resetting case, the segment descriptor referenced by selector
]value 0x18 is probably suitable for data segments, but inappropriate for
]the stack segment.

Exactly. In the passing case the selector is for a r/w data segment.
In the failing case, the selector is for an execute/read code segment.
While a code segment selector is valid for some segments, the stack segment
has some special checks:

IF SS is loaded
    THEN
        IF segment selector is NULL
            THEN #GP(0); FI;
        IF segment selector index is outside descriptor table limits
        or segment selector's RPL ≠ CPL
        or segment is not a writable data segment<=========
        or DPL ≠ CPL
            THEN #GP(selector); FI;

... So a GP fault. Weird that the load of the new GDT is getting
skipped, but stranger things have happened.
Thanks,
Scott


]Thanks
]Laszlo
]/ducks