On 02/12/2018 12:46 PM, 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:

(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: yilzh...@redhat.com
Cc: Jose Ricardo Ziviani <jos...@linux.vnet.ibm.com>
Signed-off-by: Yasmin Beatriz <yasm...@linux.vnet.ibm.com>
Signed-off-by: Daniel Henrique Barboza <danie...@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.

I am passing -ret here to provide a positive errno value to error_setg_errno. I am not sure if os_error can be negative and didn't want to risk strerror() breaking.



Daniel



Cheers
Murilo


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