[Bug lto/109368] New: LTO drops entry point symbol

[Bug lto/109368] New: LTO drops entry point symbol

[pali at kernel dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109368

            Bug ID: 109368
           Summary: LTO drops entry point symbol
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pali at kernel dot org
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---
            Target: Mingw32

LTO for PE executables drops entry point symbol when the default entry point is
used. There is no warning and just PE AddressOfEntryPoint is zeroed. Which
results in broken PE binary.

When non-default entry point is used and specified via -e option then LTO does
not drop entry point symbol and generates working PE executable.



Simple test case which does not use any system library or startup file:

$ cat test-nostartfiles.c
int mainCRTStartup(void) { return 0; }

Default console binary has entry point mainCRTStartup() function (as hardcoded
in LD sources).

$ i686-w64-mingw32-gcc -Wall -Wextra -nostartfiles -nodefaultlibs -nostdlib
test-nostartfiles.c -o test-nostartfiles.exe

Without LTO it generates working PE binary which correctly returns 0 to system.
It also has correct AddressOfEntryPoint field in PE:

$ i686-w64-mingw32-objdump -p test-nostartfiles.exe | grep AddressOfEntryPoint
AddressOfEntryPoint     00001000



When compiling with LTO it does not throw any warning but generates broken PE
binary:

$ i686-w64-mingw32-gcc -Wall -Wextra -nostartfiles -nodefaultlibs -nostdlib
test-nostartfiles.c -o test-nostartfiles.exe -flto

Trying to run it, it crashes and has zeroed AddressOfEntryPoint:

$ i686-w64-mingw32-objdump -p test-nostartfiles.exe | grep AddressOfEntryPoint
AddressOfEntryPoint     00000000



When non-default entry point is used (specified via -e option) then LTO works
correctly and does not drop its entry point.

$ cat test-nostartfiles2.c
int my_entry(void) { return 0; }

$ i686-w64-mingw32-gcc -Wall -Wextra -nostartfiles -nodefaultlibs -nostdlib -e
_my_entry test-nostartfiles2.c -o test-nostartfiles2.exe -flto

$ i686-w64-mingw32-objdump -p test-nostartfiles2.exe | grep AddressOfEntryPoint
AddressOfEntryPoint     00001000

Compiled binary works fine. 



So there is a bug in LTO compiler that it drops entry point if default one is
used (i.e. when entry point is not specified via -e option).

[Bug lto/109368] LTO drops entry point symbol

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109368

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Are you sure this is a GCC issue rather than a binutils issue?

[Bug lto/109368] LTO drops entry point symbol

[pali at kernel dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109368

--- Comment #2 from Pali Rohár <pali at kernel dot org> ---
I do not know. The issue happens when LTO is enabled for GCC.

[Bug lto/109368] LTO drops entry point symbol

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109368

Alexander Monakov <amonakov at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amonakov at gcc dot gnu.org

--- Comment #3 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
This is a Binutils bug. GCC does not have any special knowledge that
mainCRTStartup needs to be preserved (depending on targeted Windows subsystem),
it just preserves the symbols the linker tells it to preserve.

You can see how the linker selects the default entry point for PE targets:
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/emultempl/pep.em;h=2a3fd0e6ea802b3683dc786540d13b92f92e9160;hb=HEAD#l572

Perhaps the selection happens too late, even though there's a specific attempt
to take it into account:
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/ldlang.c;h=b684e2d479acf18f58076a3b5c78627caa6d8e4f;hb=HEAD#l8082

(if you inspect GCC dumps produced with -fdump-ipa-all, you'll see that
mainCRTStartup is marked as prevailing_def_ironly instead of prevailing_def)

Would you mind reporting it to the Binutils bugzilla and leaving a link to the
new bug here?

[Bug lto/109368] LTO drops entry point symbol

[pali at kernel dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109368

--- Comment #4 from Pali Rohár <pali at kernel dot org> ---
Reported to binutils: https://sourceware.org/bugzilla/show_bug.cgi?id=30300

[Bug lto/109368] LTO drops entry point symbol

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109368

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |xry111 at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |MOVED
                URL|                            |https://sourceware.org/bugz
                   |                            |illa/show_bug.cgi?id=30300

--- Comment #5 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
Mark as moved then.