[llvm-bugs] [Bug 34636] New: make -fsanitize-coverage=pc-table friendly with -ffunction-sections -Wl, -gc-sections

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34636

            Bug ID: 34636
           Summary: make -fsanitize-coverage=pc-table friendly with
                    -ffunction-sections -Wl,-gc-sections
           Product: new-bugs
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: mascasa at google.com
          Reporter: kcc at google.com
                CC: ekarpenkov at apple.com, eugeni.stepanov at gmail.com,
                    llvm-bugs at justinbogner.com, llvm-bugs at lists.llvm.org,
                    peter at pcc.me.uk

[discussed previously at
http://lists.llvm.org/pipermail/llvm-dev/2017-September/117315.html]


On linux we have two problems with -fsanitize=fuzzer and -ffunction-sections
-Wl,-gc-sections

Test for these: projects/compiler-rt/test/fuzzer/GcSectionsTest.cpp

First problem: 

% clang -std=c++11 -ffunction-sections -Wl,-gc-sections -fsanitize=fuzzer 
GcSectionsTest.cpp 
/tmp/GcSectionsTest-065286.o: In function `sancov.module_ctor':
GcSectionsTest.cpp:(.text.sancov.module_ctor[sancov.module_ctor]+0x22):
undefined reference to `__start___sancov_pcs'
GcSectionsTest.cpp:(.text.sancov.module_ctor[sancov.module_ctor]+0x2c):
undefined reference to `__stop___sancov_pcs'

Here, the table produced by -fsanitize-coverage=pc-table __sancov_pcs gets
dropped by the bfd linker (on Ubuntu 14.04). 

With a better linker (-fuse-ld=gold or -fuse-ld=lld) this doesn't happen,
but we still *may* need to find a workaround suitable for old ld. Or maybe not. 


Second:

% clang -std=c++11 -ffunction-sections -Wl,-gc-sections -fsanitize=fuzzer 
GcSectionsTest.cpp -fuse-ld=lld && nm a.out | grep Unused 
000000000023f590 t UnusedFunctionShouldBeRemovedByLinker


here, gc-sections is essentially disabled because -fsanitize-coverage=pc-table
makes all functions used. 

eugenis@ suggests that all we need here is to use
https://llvm.org/docs/LangRef.html#associated-metadata to mark the pc-table as
associated with it's function. 

This is important, we need gc-sections to work. 
But once we fix it, we'll need a test (in test/fuzzer/gc-sections.test)
but the test won't work with bfd linker, see above.

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