[llvm-bugs] [Bug 49251] New: Undefined symbol with CFI and __attribute__((__used__))

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https://bugs.llvm.org/show_bug.cgi?id=49251

            Bug ID: 49251
           Summary: Undefined symbol with CFI and
                    __attribute__((__used__))
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: tejohnson at google.com
          Reporter: samitolvanen at google.com
                CC: htmldeveloper at gmail.com, i at maskray.me,
                    llvm-bugs at lists.llvm.org, ndesaulniers at google.com,
                    peter at pcc.me.uk

Commit rG3c4c205060c9 ("[WPD][lld] Test handling of vtable definition from
shared libraries") breaks my Linux kernel build with ThinLTO and CFI, because
the compiler now generates an undefined symbol in a kernel module for a static
function with  __attribute__((__used__)).

A stand-alone reproducer:

$ cat test.c
static __attribute__((__used__)) f1() {}
f2() {}

$ clang -flto=thin -fsanitize=cfi -fvisibility=default -O2 -c -o test.o test.c
...
$ ld.lld -r -o test.lto.o test.o
$ readelf --symbols test.lto.o | grep f1
     9: 0000000000000000     1 FUNC    GLOBAL HIDDEN     4
f1$0c315d2ea609b3d583650e385e8cf855
    11: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND f1

Before this commit, the undefined symbol wasn't generated:

$ readelf --symbols test.lto.o | grep f1
     9: 0000000000000000     1 FUNC    GLOBAL HIDDEN     4
f1$0c315d2ea609b3d583650e385e8cf855

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