[PATCH v2] init: make init failures more explicit

[Michael Opdenacker]

This patch proposes to make init failures more explicit.

Before this, the "No init found" message didn't help much.
It could sometimes be misleading and actually mean
"No *working* init found".

This message could hide many different issues:
- no init program candidates found at all
- some init program candidates exist but can't be executed
  (missing execute permissions, failed to load shared libraries,
  executable compiled for an unknown architecture...)

This patch notifies the kernel user when a candidate init program is
found but can't be executed. The error code is displayed, to quickly
find the root cause. "No init found" is also replaced by
"No working init found", which is more correct.

This will help embedded Linux developers (especially the new comers),
regularly making and debugging new root filesystems.

Signed-off-by: Michael Opdenacker <michael.opdenacker@xxxxxxxxxxxxxxxxxx>
---
 init/main.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/init/main.c b/init/main.c
index 63d3e8f..3662767 100644
--- a/init/main.c
+++ b/init/main.c
@@ -811,6 +811,20 @@ static int run_init_process(const char *init_filename)
 		(const char __user *const __user *)envp_init);
 }
 
+static int try_to_run_init_process(const char *init_filename)
+{
+	int ret;
+
+	ret = run_init_process(init_filename);
+
+	if (ret && ret != -ENOENT) {
+		pr_err("Starting init: %s exists but couldn't execute it (error: %d - see errno-base.h and errno.h)\n",
+		       init_filename, ret);
+	}
+
+	return ret;
+}
+
 static noinline void __init kernel_init_freeable(void);
 
 static int __ref kernel_init(void *unused)
@@ -843,13 +857,13 @@ static int __ref kernel_init(void *unused)
 		pr_err("Failed to execute %s.  Attempting defaults...\n",
 			execute_command);
 	}
-	if (!run_init_process("/sbin/init") ||
-	    !run_init_process("/etc/init") ||
-	    !run_init_process("/bin/init") ||
-	    !run_init_process("/bin/sh"))
+	if (!try_to_run_init_process("/sbin/init") ||
+	    !try_to_run_init_process("/etc/init") ||
+	    !try_to_run_init_process("/bin/init") ||
+	    !try_to_run_init_process("/bin/sh"))
 		return 0;
 
-	panic("No init found.  Try passing init= option to kernel. "
+	panic("No working init found.  Try passing init= option to kernel. "
 	      "See Linux Documentation/init.txt for guidance.");
 }
 
-- 
1.8.1.2

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