[Qemu-devel] [PATCH] linux-user: If loading fails, print error as string, not number

[Qemu-devel] [PATCH] linux-user: If loading fails, print error as string, not number

[Peter Maydell]

If the attempt to load the guest executable fails, print the
error message as a string, not a number. This requires us to
fix a couple of places in loader_exec() where we were returning
-1 instead of a valid negative errno.

The change allows us to drop the "Unknown binary format" message
because the strerror-enhanced message is now a more self-explanatory
"Error while loading $guest-binary: Exec format error".

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
My motivation here was that "user downloads guest binary from somewhere
and it ends up without exec permissions" is quite a common mistake,
and "Error -13 while loading my-program" is not a very helpful way
to report this...

 linux-user/linuxload.c |    8 ++++----
 linux-user/main.c      |    2 +-
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/linux-user/linuxload.c b/linux-user/linuxload.c
index b47025f..381ab89 100644
--- a/linux-user/linuxload.c
+++ b/linux-user/linuxload.c
@@ -140,8 +140,9 @@ int loader_exec(const char * filename, char ** argv, char ** envp,
     bprm->p = TARGET_PAGE_SIZE*MAX_ARG_PAGES-sizeof(unsigned int);
     memset(bprm->page, 0, sizeof(bprm->page));
     retval = open(filename, O_RDONLY);
-    if (retval < 0)
-        return retval;
+    if (retval < 0) {
+        return -errno;
+    }
     bprm->fd = retval;
     bprm->filename = (char *)filename;
     bprm->argc = count(argv);
@@ -165,8 +166,7 @@ int loader_exec(const char * filename, char ** argv, char ** envp,
             retval = load_flt_binary(bprm,regs,infop);
 #endif
         } else {
-            fprintf(stderr, "Unknown binary format\n");
-            return -1;
+            return -ENOEXEC;
         }
     }
 
diff --git a/linux-user/main.c b/linux-user/main.c
index 7dea084..ae0de9a 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -3581,7 +3581,7 @@ int main(int argc, char **argv, char **envp)
     ret = loader_exec(filename, target_argv, target_environ, regs,
         info, &bprm);
     if (ret != 0) {
-        printf("Error %d while loading %s\n", ret, filename);
+        printf("Error while loading %s: %s\n", filename, strerror(-ret));
         _exit(1);
     }
 
-- 
1.7.9.5

Re: [Qemu-devel] [PATCH] linux-user: If loading fails, print error as string, not number

[malc]

On Fri, 24 Aug 2012, Peter Maydell wrote:

> If the attempt to load the guest executable fails, print the
> error message as a string, not a number. This requires us to
> fix a couple of places in loader_exec() where we were returning
> -1 instead of a valid negative errno.
> 
> The change allows us to drop the "Unknown binary format" message
> because the strerror-enhanced message is now a more self-explanatory
> "Error while loading $guest-binary: Exec format error".

Double edged sword:

a. Localized strings that is near impossible to map back to anything
   useful
b. Codes whose meaning is overloaded beyond repair

Sometimes sticking to plain numbers works better.

[..snip..]

-- 
mailto:av1474@comtv.ru

Re: [Qemu-devel] [PATCH] linux-user: If loading fails, print error as string, not number

[Andreas Färber]

Am 24.08.2012 20:07, schrieb malc:
> On Fri, 24 Aug 2012, Peter Maydell wrote:
> 
>> If the attempt to load the guest executable fails, print the
>> error message as a string, not a number. This requires us to
>> fix a couple of places in loader_exec() where we were returning
>> -1 instead of a valid negative errno.
>>
>> The change allows us to drop the "Unknown binary format" message
>> because the strerror-enhanced message is now a more self-explanatory
>> "Error while loading $guest-binary: Exec format error".
> 
> Double edged sword:
> 
> a. Localized strings that is near impossible to map back to anything
>    useful
> b. Codes whose meaning is overloaded beyond repair
> 
> Sometimes sticking to plain numbers works better.

I ran into the same issue some time ago and was lucky to have pm215 on
#qemu knowing what was causing this. A pure number (or number translated
to E*) is not so telling.

Could the diplomatic solution be to include the number in the error
message? :)

Andreas

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