Messages in this thread | | | Date | Thu, 8 Nov 2018 14:00:54 -0800 | From | Matthew Wilcox <> | Subject | Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack |
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On Thu, Nov 08, 2018 at 01:48:54PM -0800, Dave Hansen wrote: > On 11/8/18 1:22 PM, Andy Lutomirski wrote: > >> +struct cet_kernel_state { > >> + u64 kernel_ssp; /* kernel shadow stack */ > >> + u64 pl1_ssp; /* ring-1 shadow stack */ > >> + u64 pl2_ssp; /* ring-2 shadow stack */ > >> +} __packed; > >> + > > Why are these __packed? It seems like it'll generate bad code for no > > obvious purpose. > > It's a hardware-defined in-memory structure. Granted, we'd need a > really wonky compiler to make that anything *other* than a nicely-packed > 24-byte structure, but the __packed makes it explicit. > > It is probably a really useful long-term thing to stop using __packed > and start using "__hw_defined" or something that #defines down to __packed.
packed doesn't mean "don't leave gaps". It means:
'packed' The 'packed' attribute specifies that a variable or structure field should have the smallest possible alignment--one byte for a variable, and one bit for a field, unless you specify a larger value with the 'aligned' attribute.
So Andy's right. It tells the compiler, "this struct will not be naturally aligned, it will be aligned to a 1-byte boundary". Which is silly. If we have
struct b { unsigned long x; } __packed;
struct a { char c; struct b b; };
we want struct b to start at offset 8, but with __packed, it will start at offset 1.
Delete __packed. It doesn't do what you think it does.
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