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 <yasm...@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

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