[llvm-dev] Usage of the jumptable attribute
via llvm-dev
llvm-dev at lists.llvm.org
Wed Sep 4 06:58:46 PDT 2019
> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Krzysztof Parzyszek via llvm-dev
> Sent: Wednesday, September 04, 2019 9:08 AM
> To: CompilerCrash; llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] Usage of the jumptable attribute
>
> Hi Daniel,
> This attribute doesn't do anything anymore. It was added a while back to
> implement control flow integrity, but that work was eventually removed. I
> believe it's still there for backwards compatibility (i.e. to read old
> bitcode).
In that case, shouldn't you update the LangRef to say so?
--paulr
>
> --
> Krzysztof Parzyszek kparzysz at quicinc.com AI tools development
>
> > -----Original Message-----
> > From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of
> CompilerCrash
> > via llvm-dev
> > Sent: Saturday, August 31, 2019 2:22 AM
> > To: llvm-dev at lists.llvm.org
> > Subject: [EXT] [llvm-dev] Usage of the jumptable attribute
> >
> > Hello everyone,
> >
> > I'm new to LLVM (which is a really great project by the way) and I hope
> this
> > is the right place for my question.
> >
> > In the LLVM Language Reference Manual I found the "jumptable" function
> > attribute, which seems to be exactly what I need for my project (where I
> want
> > to add one level of indirection to every function call), but I have
> trouble
> > figuring out, how to use it correctly.
> >
> > I wrote my own transformation pass (using LLVM 8.0.1) to add this
> attribute
> > as well as unnamed_addr (I'm aware that this might break some programs,
> but
> > jumptable requires it) to every function definition:
> >
> > // includes ...
> >
> > using namespace llvm;
> >
> > namespace {
> > struct MyPass : public ModulePass {
> > static char ID;
> > MyPass() : ModulePass(ID) {}
> >
> > bool runOnModule(Module &M) override {
> > for (Function &F : M.getFunctionList()) {
> > if (!F.isDeclaration()) {
> > F.setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
> > F.addFnAttr(Attribute::JumpTable);
> > }
> > }
> > return true;
> > }
> > };
> > }
> >
> > char MyPass::ID = 0;
> > static RegisterPass<MyPass> X("mypass", "My Pass");
> >
> > And I wrote a program test.c:
> >
> > #include <stdio.h>
> >
> > void foo() {
> > puts("Hello World");
> > }
> >
> > int main() {
> > foo();
> > }
> >
> > I compiled it to test.ll and ran it through opt:
> >
> > clang -O0 -emit-llvm -S -o test.ll test.c
> > build/bin/opt -load build/lib/LLVMMyPass.so -mypass -S -o
> test.out.ll
> > test.ll
> >
> > The output (test.out.ll) was identical to test.ll except for the 3
> > occurrences of jumptable and the last 2 occurrences of unnamed_addr:
> >
> > ; ModuleID = 'test.ll'
> > source_filename = "test.c"
> > target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
> > target triple = "x86_64-unknown-linux-gnu"
> >
> > @.str = private unnamed_addr constant [12 x i8] c"Hello World\00",
> align
> > 1
> >
> > ; Function Attrs: jumptable noinline nounwind optnone uwtable
> > define dso_local void @foo() unnamed_addr #0 {
> > %1 = call i32 @puts(i8* getelementptr inbounds ([12 x i8], [12 x
> > i8]* @.str, i32 0, i32 0))
> > ret void
> > }
> >
> > declare dso_local i32 @puts(i8*) #1
> >
> > ; Function Attrs: jumptable noinline nounwind optnone uwtable
> > define dso_local i32 @main() unnamed_addr #0 {
> > call void @foo()
> > ret i32 0
> > }
> >
> > attributes #0 = { jumptable noinline nounwind optnone uwtable "no-
> jump-
> > tables"="false"
> > ; ... many more attributes ...
> > }
> > attributes #1 = {
> > ; ... many more attributes ...
> > }
> >
> > !llvm.module.flags = !{!0}
> > !llvm.ident = !{!1}
> >
> > !0 = !{i32 1, !"wchar_size", i32 4}
> > !1 = !{!"clang version 8.0.0 (tags/RELEASE_800/final)"}
> >
> > Finally, I executed:
> >
> > build/bin/llc -o test.out.s test.out.ll
> > clang -o test.out.native test.out.s
> > objdump --disassemble-all test.out.native
> >
> > I hoped to see some kind of jump table being used for the call to foo
> (maybe
> > even the plt mechanism the dynamic library calls use), but it is just a
> > "normal" jump to foo's address:
> >
> > 0000000000400500 <foo>:
> > 400500: 55 push %rbp
> > 400501: 48 89 e5 mov %rsp,%rbp
> > 400504: 48 bf c0 05 40 00 00 movabs $0x4005c0,%rdi
> > 40050b: 00 00 00
> > 40050e: e8 cd fe ff ff callq 4003e0 <puts at plt>
> > 400513: 5d pop %rbp
> > 400514: c3 retq
> > 400515: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
> > 40051c: 00 00 00
> > 40051f: 90 nop
> >
> > 0000000000400520 <main>:
> > 400520: 55 push %rbp
> > 400521: 48 89 e5 mov %rsp,%rbp
> > 400524: e8 d7 ff ff ff callq 400500 <foo>
> > 400529: 31 c0 xor %eax,%eax
> > 40052b: 5d pop %rbp
> > 40052c: c3 retq
> > 40052d: 0f 1f 00 nopl (%rax)
> >
> > So, what am I doing wrong? The documentation indicates that the jump
> table is
> > created at code-generation time. So I guess my pass works fine and I
> just
> > need to get the code generator to process the jumptable attribute
> somehow? I
> > found the createJumpInstrTablesPass in include/CodeGen/Passes.h, but it
> seems
> > like it's never used.
> >
> > Thanks in advance
> >
> > Daniel
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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