[llvm-dev] [RFC] CFI for indirect calls with ThinLTO

Mon May 15 18:44:28 PDT 2017

Thanks for sending this out. A few comments below.

On Mon, May 15, 2017 at 5:17 PM, Evgenii Stepanov via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> this is a proposal for the implementation of CFI-icall [1] with ThinLTO.
>
> Jumptables are generated in the merged module. To generate a
> jumptable, we need a list of functions with !type annotations,
> including (in non-cross-dso mode) external functions. Unfortunately,
> LLVM IR does not preserve unused function declarations, and we don’t
> want to copy the actual function bodies to the merged module.
>
> Indirect call targets can be represented in the following way using
> named metadata:
>
> void foo() {}
> int bar() { return 0; }
>
> # Merged module
> !cfi.functions = !{!1, !3}
> !1 = !{!"bar", i8 0, !2}
> !2 = !{i64 0, !"_ZTSFiE"}
> !3 = !{!"foo", i8 0, !4}
> !4 = !{i64 0, !"_ZTSFvE"}
>

Presumably there would be no entries in !cfi.functions for functions
defined in the merged module, as the type metadata would come from the
module itself.

>
> Each function is described by a tuple of
> * Promoted name as a string
>

I imagine that we would only promote a function if it is address-taken.
Otherwise we could be inhibiting optimization significantly.

* Linkage (see below)
> * Type(s)
>
>
> A function can have multiple types. In the Cross-DSO mode each
> function has a second “external” numeric type, and we might want to
> allow “relaxed” type checking in the future where a function could
> conform to multiple types. In that case the metadata would look like
> this:
>
> !4 = !{!"bar", i8 0, !5, !6}
> !5 = !{i64 0, !"_ZTSFiE"}
> !6 = !{i64 0, i64 751454132325070187}
>
> “Linkage” is one of: definition, external declaration, external weak
> declaration.
>
> In the merged “merged” module, !cfi.functions may contain multiple
> entries for each function. We pick one with the strongest linkage
> (i.e. the definition, if it is available) in LowerTypeTests.
>

It's unfortunate that this design effectively requires that the
LowerTypeTests pass recompute the linkage for each symbol, as the linker
already knows this information (and could, in principle, provide it to the
pass). But I'm not sure if there's a better way to do it.

>
> The LTO step emits, for a defined function named “f”:
> declare void f.cfi()
> .jumptable:
>     …
>     call f.cfi
>     ...
> f.cfi-jt = alias .jumptable + offset
> f = alias f.cfi-jt
>
> The same for an external (either weak or strong) declaration of a
> function named “f”:
> .jumptable:
>     …
>     call f
>     ...
> f.cfi-jt = alias .jumptable + offset
>
>
One thing to be careful about is summary-based dead stripping: the pass
needs to be able to query whether any specific function is still live in
order to avoid introducing undefined symbol references. I think we can do
that by adding a Live flag to GlobalValueSummaryInfo (which I think should
also let us fix a number of FIXMEs elsewhere, e.g. http://llvm-cs.pcc.me.uk/
lib/Transforms/IPO/LowerTypeTests.cpp#1447 http://llvm-cs.pc
c.me.uk/lib/Transforms/IPO/WholeProgramDevirt.cpp#1329), and have the pass
check the flag for each function.

Weak external linkage is used in the lowering of uses of @f. This is
> done both in the merged module and in ThinLTO backends. Uses of strong
> definitions and declarations are replaced with f.cfi-jt. Uses of weak
> external declarations a replaced with (f ? f.cfi-jt : 0) instead.

> ThinLTO backends need to know which functions have jumptable entries
> created for them (they will need to be RAUWed with f.cfi-jt). In the
> Cross-DSO mode, external functions don’t get jumptable entries. This
> information is passed back from the LTO step through combined summary.
> The current idea is to add a new record, FunctionTypeResolution, which
> would contain a set of function names in the jumptable.
>

It occurred to me that this design could prevent inlining of indirect calls
via constant propagation. For example, suppose that we have a module that
looks like this:

define void @f() {
  ret void
}

define void @g() {
  %fp = call i8* @identity(i8* @f)
  call void %fp()
}

and a second module:

define i8* @identity(i8* %ptr) {
  return %ptr
}

and @identity is imported into the first module. Now I think the first
module would look like this after optimization:

define void @f.cfi() {
  ret void
}

declare void @f.cfi-jt()

define void @g.cfi() {
  call void @f.cfi-jt()
}

So we cannot inline f.cfi into g.cfi, as the optimizer does not know that
f.cfi-jt can be replaced with f.cfi. I'm not sure how likely this would be
in practice, but something to keep in mind.

Peter

> == Alternatives
>
> Function type information can be passed in the summary, as a list of
> records (name, linkage, type(, type)*).
> * Type can be either a string or a number. This complicates the encoding.
> * The code in LowerTypeTests works with !type metadata in the same
> format as described above. It would need to either recreate the
> metadata from the summary, or deal with different input formats.
> I don’t see any advantages to this encoding. Could it be more compact
> than the metadata approach?
>
> [1] https://clang.llvm.org/docs/ControlFlowIntegrity.html#indire
> ct-function-call-checking
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>

-- 
-- 
Peter
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