Re: [Qemu-devel] [PATCH v3 1/1] dump.c: allow fd_write_vmcore to return errno on failure

[Murilo Opsfelder Araujo <muriloo@linux.vnet.ibm.com>]

On 02/12/2018 12:25 PM, Daniel Henrique Barboza wrote:
> From: Yasmin Beatriz <yasmins@linux.vnet.ibm.com>
> 
> fd_write_vmcore can fail to execute for a lot of reasons that can be
> retrieved by errno, but it only returns -1. This makes difficult for
> the caller to know what happened and only a generic error message is
> propagated back to the user. This is an example using dump-guest-memory:
> 
> (qemu) dump-guest-memory /home/yasmin/mnt/test.dump
> dump: failed to save memory
> 
> All callers of fd_write_vmcore of dump.c does error handling via
> error_setg(), so at first it seems feasible to add the Error pointer as
> an argument of fd_write_vmcore. This proved to be more complex than it
> first looked. fd_write_vmcore is used by write_elf64_notes and
> write_elf32_notes as a WriteCoreDumpFunction prototype. WriteCoreDumpFunction
> is declared in include/qom/cpu.h and is used all around the code. This
> leaves us with few alternatives:
> 
> - change the WriteCoreDumpFunction prototype to include an error pointer.
> This would require to change all functions that implements this prototype
> to also receive an Error pointer;
> 
> - change both write_elf64_notes and write_elf32_notes to no use the
> WriteCoreDumpFunction. These functions use not only fd_write_vmcore
> but also buf_write_note, so this would require to change buf_write_note
> to handle an Error pointer. Considerable easier than the alternative
> above, but it's still a lot of code just for the benefit of the callers
> of fd_write_vmcore.
> 
> This patch presents an easier solution that benefits all fd_write_vmcore
> callers:
> 
> - instead of returning -1 on error, return -errno. All existing callers
> already checks for ret < 0 so there is no need to change the caller's
> logic too much. This also allows the retrieval of the errno.
> 
> - all callers were updated to use error_setg_errno instead of just
> errno_setg. Now that fd_write_vmcore can return an errno, let's update
> all callers so they can benefit from a more detailed error message.
> 
> This is the same dump-guest-memory example with this patch applied:
> 
> (qemu) dump-guest-memory /home/yasmin/mnt/test.dump
> dump: failed to save memory: No space left on device
> (qemu)
> 
> This example illustrates an error of fd_write_vmcore when called
> from write_data. All other callers will benefit from better
> error messages as well.
> 
> Reported-by: yilzhang@redhat.com
> Cc: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
> Signed-off-by: Yasmin Beatriz <yasmins@linux.vnet.ibm.com>
> Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
> ---
>  dump.c | 23 ++++++++++++++---------
>  1 file changed, 14 insertions(+), 9 deletions(-)
> 
> diff --git a/dump.c b/dump.c
> index 7b13baa413..171ff8a3b8 100644
> --- a/dump.c
> +++ b/dump.c
> @@ -107,7 +107,7 @@ static int fd_write_vmcore(const void *buf, size_t size, void *opaque)
> 
>      written_size = qemu_write_full(s->fd, buf, size);
>      if (written_size != size) {
> -        return -1;
> +        return -errno;
>      }
> 
>      return 0;
> @@ -140,7 +140,7 @@ static void write_elf64_header(DumpState *s, Error **errp)
> 
>      ret = fd_write_vmcore(&elf_header, sizeof(elf_header), s);
>      if (ret < 0) {
> -        error_setg(errp, "dump: failed to write elf header");
> +        error_setg_errno(errp, -ret, "dump: failed to write elf header");

Do we need -ret passed to error_setg_errno()? fd_write_vmcore() returns
negative errno in case of error.

Cheers
Murilo