Il 12/02/23 07:15, Flavio Cruz ha scritto:
Made changes to MIG so that generated stubs can be compiled when using
a 64 bit userland. The most important change here is to ensure
that data is always 8-byte aligned. Not doing that will lead to
undefined behavior since on 64 bits many structures can contain 64 bit
scalars which require 8-byte alignment. Even if x86 is usually fine with
unaligned accesses, compilers are taking advantage of this UD which may
break us in the future
(http://pzemtsov.github.io/2016/11/06/bug-story-alignment-on-x86.html)
The downside is that messages get significantly larger since we have
to pad the initial mach_msg_type_t so that the data item can be 8-byte
aligned. In the future, I want to make MIG layout the data in a way
that non-complex data can be packed together (essentially, how XNU messages
work) and to force userland to create larger structures so that we don't
have to do any structure resizing while in the kernel. For now, this unblocks
future work on 64 bits and is quite simple to understand.
For the 64 bit / 32 bit configuration, we can pass --enable-user32 when
targeting x86_64, and MIG will generate stubs as before. I am unsure
whether there exists undefined behavior in that particular configuration
since most of the structures used by the kernel do not seem to use 8
byte scalars so potentially we should be OK there.
As mentioned in the article, using memcpy() to a correctly aligned
pointer seems to avoid the possible alignment issues. Since in the
kernel we need to use copyin/copyout anyway, wouldn't this be enough?
As for the mig stubs, if I understand correctly the issue stems from the
fact that they use a struct to create the message fields in memory, so
the binary format of a message needs to have the same alignment
requirements. Would using memcpy here avoid the alignment issue?
I think it would even simplify the stub code, e.g. for array types, and
I'm not sure it would decrease performance.
As a follow up, we will need to change gnumach to iterate over these
messages correctly (to follow the 8-byte alignment rules).
I see that currently even the 64-bit version is compiled with
-mno-3dnow -mno-mmx -mno-sse -mno-sse2, do you mean to also remove these
flags?
diff --git a/utils.h b/utils.h
index 4be4f4c..477ea74 100644
--- a/utils.h
+++ b/utils.h
@@ -39,6 +39,8 @@
({ __typeof__ (a) _a = (a); \
__typeof__ (b) _b = (b); \
_a > _b ? _b : _a; })
+#define ALIGN(x, alignment) \
+ ((x) + (alignment)-1) & ~((alignment)-1)
Maybe this could be replaced by the mach_msg_align() macro from
mach/message.h. As far as I understand, they have similar purpose, and
it may already include the USER32 setting, in the future.
Luca
