Re: [PATCH] contrib/elf2dmp: a workaround for the buggy msvcrt.dll!fwrite

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Jul 08, 2024 at 07:25:20PM +0800, junjiehua wrote:
> when building elf2dump with x86_64-w64-mingw32-gcc, fwrite is imported from
> msvcrt.dll. However, the implementation of msvcrt.dll!fwrite is buggy:
> it enters an infinite loop when the size of a single write exceeds 4GB.
> This patch addresses the issue by splitting large physical memory
> blocks into smaller chunks.
> 
> Signed-off-by: junjiehua <junjiehua@tencent.com>
> ---
>  contrib/elf2dmp/main.c | 19 +++++++++++++++----
>  1 file changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/contrib/elf2dmp/main.c b/contrib/elf2dmp/main.c
> index d046a72ae6..1994553d95 100644
> --- a/contrib/elf2dmp/main.c
> +++ b/contrib/elf2dmp/main.c
> @@ -23,6 +23,8 @@
>  #define INITIAL_MXCSR   0x1f80
>  #define MAX_NUMBER_OF_RUNS  42
>  
> +#define MAX_CHUNK_SIZE (128 * 1024 * 1024)
> +
>  typedef struct idt_desc {
>      uint16_t offset1;   /* offset bits 0..15 */
>      uint16_t selector;
> @@ -434,13 +436,22 @@ static bool write_dump(struct pa_space *ps,
>  
>      for (i = 0; i < ps->block_nr; i++) {
>          struct pa_block *b = &ps->block[i];
> +        size_t offset = 0;
> +        size_t chunk_size;
>  
>          printf("Writing block #%zu/%zu of %"PRIu64" bytes to file...\n", i,
>                  ps->block_nr, b->size);
> -        if (fwrite(b->addr, b->size, 1, dmp_file) != 1) {
> -            eprintf("Failed to write block\n");
> -            fclose(dmp_file);
> -            return false;
> +
> +        while (offset < b->size) {
> +            chunk_size = (b->size - offset > MAX_CHUNK_SIZE)
> +                         ? MAX_CHUNK_SIZE
> +                         : (b->size - offset);
> +            if (fwrite(b->addr + offset, chunk_size, 1, dmp_file) != 1) {
> +                eprintf("Failed to write block\n");
> +                fclose(dmp_file);
> +                return false;
> +            }
> +            offset += chunk_size;
>          }
>      }

When reading the original ELF file, we don't actually fread() it,
instead we mmap it, using GMappedFile on Windows. Rather than
working around fwrite() bugs, we could do the same for writing
and create a mapped file and just memcpy the data across.


With regards,
Daniel
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