https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=279501
Bug ID: 279501
Summary: linuxulator: amd64 Global Descriptor Table ABI
incompatibility
Product: Base System
Version: Unspecified
Hardware: amd64
OS: Any
Status: New
Severity: Affects Only Me
Priority: ---
Component: kern
Assignee: [email protected]
Reporter: [email protected]
Created attachment 251198
--> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=251198&action=edit
Minimal test of executing 32bit code from 64bit process
On amd64, a program may jump between 32bit and 64bit code using far returns to
set the code segment register. The mechanism is OS-dependent since it depends
on the layout of the Global Descriptor Table initialized by the kernel. It is
used for example for a 64bit program to utilize 32bit assembly libraries or for
a 32bit compatibility layer implemented using 64bit libraries as with WINE's
WOW64.
Currently this mechanism is broken in the linux compatibility layer: whereas
the program expects a Linux GDT, it is the FreeBSD GDT in effect. Also, Linux
program code reading the cs register gets different results than expected: 0x23
vs 0x33 for 32bit, 0x33 vs 0x43 for 64bit.
$ yasm -fbin -o test testgdt.asm
produces a FreeBSD executable which writes "A", then writes "B", then exits.
$ yasm -fbin -o test testgdt.asm -DLINUX -DLINUX_GDT
produces a Linux executable with the expected result only when run on Linux
x86_64.
On FreeBSD it writes "A" before dying with a bus error.
$ yasm -fbin -o test testgdt.asm -DLINUX
produces an executable which writes the "expected result" only when run on
FreeBSD amd64 with linux compatibility.
On Linux it writes "A" before dying with a segfault.
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