Segfault when using C compatible variadic functions #58980

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1. dlrobertson commented on Mar 7, 2019

   

   dlrobertson

   on Mar 7, 2019

   Contributor

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   @thedataking thanks for reporting this. The example should probably look more like the following. The example still segfaults (even in debug mode).

   #![feature(c_variadic)]
   use std::ffi::VaList;
   
   extern "C" {
       // vprintf signature is `int vprintf(const char * format, va_list arg)`
       #[no_mangle]
       fn vprintf(_: *const libc::c_char, _: VaList) -> libc::c_int;
   }
   
   #[no_mangle]
   unsafe extern "C" fn variadic(fmt: *const libc::c_char, a: ...) -> libc::c_int {
       // `va_start` in theory should be automatically called giving you a
       // `VaList` at `a` here.
       vprintf(fmt, a)
       // `va_end` should be called here.
   }
   
   fn main() {
       let fmt = std::ffi::CString::new("test %d %d %d\n").expect("new failed");
       unsafe {
           variadic(fmt.as_ptr(), 1, 2, 3);
       }
   }

   If a is used in the function variadic the issue does not happen. Essentially we're optimizing variadic to the following

   define i32 @variadic(i8* %fmt, ...) unnamed_addr #3 personality i32 (i32, i32, i64, %"unwind::libunwind::_Unwind_Exception"*, %"unwind::libunwind::_Unwind_Context"*)* @rust_eh_personality {
   start:
     %personalityslot = alloca { i8*, i32 }, align 8
     %0 = invoke i32 @vprintf(i8* %fmt, i64* align 8 dereferenceable(24) undef)
             to label %bb2 unwind label %cleanup
     ...
   

   Note the missing va_start and va_end. I think this is happening because we're hitting src/librustc_codegen_ssa/mir/mod.rs:536-540. I'll do some more investigation shortly.
2. dlrobertson commented on Mar 7, 2019

   

   dlrobertson

   on Mar 7, 2019

   Last edited by dlrobertson

   Contributor

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   @thedataking the code I list in the above example should work after #58985 is merged.

   👍React with 👍1Reacted by Per Larsen
3. jonas-schievink

   added

   A-FFIArea: Foreign function interface (FFI)Area: Foreign function interface (FFI)

   T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

   C-bugCategory: This is a bug.Category: This is a bug.

   on Mar 7, 2019
4. bors

   added a commit that references this issue on Mar 8, 2019

   Auto merge of #58985 - dlrobertson:fix_58980, r=alexreg

   ...

   This issue will close once commit b2ea6c8 is merged into the 'main' branch.

   b2ea6c8
5. bors

   closed this as completedin #58985on Mar 9, 2019