Debug is enabled but there is no output #273

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1. Rlyown commented on May 7, 2024

   

   Rlyown

   on May 7, 2024

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   Finally I fixed my bugs by printf() debugging. But I still confused, the debug flag does not take effect.
2. thomasten commented on May 7, 2024

   

   thomasten

   on May 7, 2024

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   Hi, I think CapturePFGPExceptions=1 doesn't work for all types of exceptions. Now that you know the bug, can you provide a minimal reproduction so that we can check if this is the case here?
   For debugging, you could have used ego-gdb, which should stop when the exception happens.
3. Rlyown commented on May 8, 2024

   

   Rlyown

   on May 8, 2024

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   Minimal sample code

   I called a function from a library written in C using cgo. This function caused the program to crash. It allocates a block of memory via mmap and then writes the base address of that memory to the GS register.

   It was precisely this action of writing to the GS register that led to the "ERROR: signal: segmentation fault (core dumped)". The program ran normally after I removed the line os_writegsbase(new_mem);.

   This is a sample.

   package main
   
   import "fmt"
   
   /*
   #include <fcntl.h>
   #include <stdint.h>
   #include <stdio.h>
   #include <sys/mman.h>
   #include <unistd.h>
   
   // write base addr of memory to GS segment register
   #define os_writegsbase(base_addr)                                 \
       do {                                                          \
           uint64_t __gs_value = (uint64_t)(uint64_t *)base_addr;    \
           asm volatile("wrgsbase %0" ::"r"(__gs_value) : "memory"); \
       } while (0)
   
   int mmap_function() {
     uint64_t new_size = 128 * 1024 * 1024; // 128 MB
   
     void *new_mem =
         mmap(NULL, new_size, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
   
   #if defined(os_writegsbase)
     // the code crashed here in sgx mode
     os_writegsbase(new_mem);
   #endif
   
     if (new_mem == MAP_FAILED) {
       perror("mmap error");
       return -1;
     }
   
     munmap(new_mem, new_size);
   
     return 0;
   }
   */
   import "C"
   
   // the wrapper to call function in C
   func CallMmapFunction() int {
           res := C.mmap_function()
           return int(res)
   }
   
   func main() {
           res := CallMmapFunction()
           fmt.Println(res)
   }

   To reproduce

   The enclave.json is the same as the one I mentioned above.

   1. ego-go build test.go
   2. ego sign
   3. ego run test
4. Rlyown commented on May 8, 2024

   

   Rlyown

   on May 8, 2024

   Last edited by Rlyown

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   Sorry I don't know much about open enclave or edgelessrt. I tried ego-gdb, but the output in this case is a bit hard to understand for me. Judging from the backtrace results, it seems that where it stops is not quite consistent with my expectations.

   

   Sometimes when I rerun ego-gdb test, it gives me a different output.

   
5. thomasten commented on May 11, 2024

   

   thomasten

   on May 11, 2024

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   Regarding SIGILL, see https://docs.edgeless.systems/ego/workflows/debug#inside-an-enclave

   Thanks for the sample code. This can be tricky to debug. The gs register is used by the SGX runtime. If it is modified by the app, the runtime will most probably crash the next time it reads the register and uses the value. You can see the location of the crash in ego-gdb, but it's not obvious what caused it.