Windows DllMain name is exported

[MauriceKayser]

On Windows, Rust exports the name of DllMain (std) / _DllMainCRTStartup (no_std), which is not standard behaviour and unwanted in most cases (that I am aware of). As far as I understand this happens because they must be marked with either #[no_mangle] or #[export_name = ".."] so the link.exe can find the symbol. The extern keyword must also be used to mark them as stdcall for 32 bit targets.

Example project:

• Cargo.toml

  [lib]
  crate-type = ["cdylib"]

• lib.rs (std)

  #[no_mangle]
  extern "system" fn DllMain(_: *const u8, _: u32, _: *const u8) -> u32 { 1 }

• lib.rs (no_std)

  #![no_std]
  
  #[panic_handler]
  fn panic(_: &core::panic::PanicInfo) -> ! { loop {} }
  
  #[no_mangle]
  extern "system" fn _DllMainCRTStartup(_: *const u8, _: u32, _: *const u8) -> u32 { 1 }

dumpbin output

• Example Windows dll file:

  Dump of file C:\Windows\System32\httpprxp.dll
  
  File Type: DLL
  
    Section contains the following exports for httpprxp.dll
  
      00000000 characteristics
          0.00 version
             1 ordinal base
             8 number of functions
             8 number of names
  
      ordinal hint RVA      name
  
            1    0 00001870 ProxyHelperGetProxyEventInformation
            2    1 00001D00 ProxyHelperProviderConnectToServer
            3    2 00001DC0 ProxyHelperProviderDisconnectFromServer
            4    3 000010E0 ProxyHelperProviderFreeMemory
            5    4 00001220 ProxyHelperProviderRegisterForEventNotification
            6    5 00001AC0 ProxyHelperProviderSetProxyConfiguration
            7    6 00001BA0 ProxyHelperProviderSetProxyCredentials
            8    7 00001620 ProxyHelperProviderUnregisterEventNotification
  
    Summary
  
          1000 .data
          1000 .didat
          1000 .pdata
          2000 .rdata
          1000 .reloc
          1000 .rsrc
          2000 .text
  

• Rust example from above (std):

  Dump of file std.dll
  
  File Type: DLL
  
    Section contains the following exports for std.dll
  
      00000000 characteristics
          0.00 version
             1 ordinal base
             2 number of functions
             2 number of names
  
      ordinal hint RVA      name
  
            1    0 00001000 DllMain = DllMain
            2    1 00001010 rust_eh_personality = rust_eh_personality
  
    Summary
  
          1000 .data
          1000 .pdata
          1000 .rdata
          1000 .reloc
          1000 .text
  

• Rust example from above (no_std):

  Dump of file no_std.dll
  
  File Type: DLL
  
    Section contains the following exports for no_std.dll
  
      00000000 characteristics
          0.00 version
             1 ordinal base
             1 number of functions
             1 number of names
  
      ordinal hint RVA      name
  
            1    0 00001000 _DllMainCRTStartup = _DllMainCRTStartup
  
    Summary
  
          1000 .rdata
          1000 .text
  

Expected output

DllMain and _DllMainCRTStartup should not be exported by name, just as in the first dumpbin output example of a Windows system dll.

Tested compiler version

rustc 1.41.0-nightly (c8ea4ace9 2019-12-14)
binary: rustc
commit-hash: c8ea4ace9213ae045123fdfeb59d1ac887656d31
commit-date: 2019-12-14
host: x86_64-pc-windows-msvc
release: 1.41.0-nightly
LLVM version: 9.0

[MSxDOS]

I think the main issue here is that #[no_mangle] also exports the function instead of just demangling it which is unexpected and unintuitive. As it seems, the only solution we currently have is to use a custom empty DEF file. Note that even then the MSVC linker will still create an empty export directory. That's not the case with LLD-linker, though.

[MauriceKayser]

I just had a short look because I wanted to try two ideas:

1. Simply blacklist the two mentioned names and do not add them to the DEF file.
2. Add some kind of optional #[no_mangle(export = "false")] mechanism.

To start off I tried modifying fn contains_extern_indicator(&self) -> bool in lib/rustlib/src/rust/src/librustc/hir/mod.rs to always return false and compiled with xbuild, but my change does not seem to be used by that. I guess I would need to recompile rustc, not core.

I can neither find any references to fn contains_extern_indicator(&self) -> bool, nor can I find the logic where the no_mangle attribute is actually parsed.

[ma6254]

I desperately need some mechanism:
Perform some initialization actions (such as network, configuration files, etc.) before running the export function after the dll is loaded by the main program

[lz520520]

Is there any solution to this problem?

[wyxather]

i've also notice rust seems to put my package name in rdata section, is there anyway to prevent it? i couldn't find any info about it other than removing panic strings / path which doesnt solve my issue.

[leo-liu]

A workaround is to write your own foo.def and pass /DEF:foo.def to the linker.

[MauriceKayser]

A workaround is to write your own foo.def and pass /DEF:foo.def to the linker.

See the first reply in this issue for caveats.