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

[Eric Blake <eblake@redhat.com>]

On 02/12/2018 08:46 AM, Murilo Opsfelder Araujo wrote:
> 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:
>>

>> +++ 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.

Yes, this usage is correct.  error_setg_errno() takes a positive errno 
value (using strerror, which only decodes positive values into useful 
strings); but we typically return negative errno values (as was 
correctly done in fd_write_vmcore), so the extra layer of negation here 
is needed.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org