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Date:   Thu, 17 Sep 2020 18:25:37 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Xiongfeng Wang <wangxiongfeng2@huawei.com>
Cc:     mchehab@kernel.org, tony.luck@intel.com,
        linux-edac@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] EDAC/mc_sysfs: Add missing newlines when printing
 {max,dimm}_location
Message-ID: <20200917162537.GL31960@zn.tnic>
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 <20200916170052.GO2643@zn.tnic>
 <591e613e-0b53-028f-08fd-3d62a35b8c4f@huawei.com>
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On Thu, Sep 17, 2020 at 07:38:57PM +0800, Xiongfeng Wang wrote:
> I am not sure if snprintf will return a value larger than its second input
> paramter 'size'.

The comment over snprintf() says

 * The return value is the number of characters which would be
 * generated for the given input, excluding the trailing null,
 * as per ISO C99.  If the return is greater than or equal to
 * @size, the resulting string is truncated.

And let's try it, see diff at the end. Now look what that produces:

[    2.594796] kernel_init: len: 16, str: [A lo]

it returns 16 for len even though the buffer is 5 chars long. So in our
patch, we'd increment by 16 which would be wrong.

Now let's use scnprintf():

[    2.700142] kernel_init: len: 4, str: [A lo]

Much better. Lemme do that.

> Not sure whether we need to check 'len' equals to 0.
> if (len <= 0) ?

Yeah, lemme fix that too, so we have now incrementally ontop:

---
diff --git a/drivers/edac/edac_mc_sysfs.c b/drivers/edac/edac_mc_sysfs.c
index fa0551c81e63..c56e0004b39e 100644
--- a/drivers/edac/edac_mc_sysfs.c
+++ b/drivers/edac/edac_mc_sysfs.c
@@ -822,18 +822,17 @@ static ssize_t mci_max_location_show(struct device *dev,
 	int i, n;
 
 	for (i = 0; i < mci->n_layers; i++) {
-		n = snprintf(p, len, "%s %d ",
-			     edac_layer_name[mci->layers[i].type],
-			     mci->layers[i].size - 1);
-
+		n = scnprintf(p, len, "%s %d ",
+			      edac_layer_name[mci->layers[i].type],
+			      mci->layers[i].size - 1);
 		len -= n;
-		if (len < 0)
+		if (len <= 0)
 			goto out;
 
 		p += n;
 	}
 
-	p += snprintf(p, len, "\n");
+	p += scnprintf(p, len, "\n");
 out:
 	return p - data;
 }
---

Test diff:

---
diff --git a/init/main.c b/init/main.c
index ae78fb68d231..e2d6110d3a3d 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1397,7 +1397,8 @@ void __weak free_initmem(void)
 
 static int __ref kernel_init(void *unused)
 {
-	int ret;
+	char str[5];
+	int ret, len;
 
 	kernel_init_freeable();
 	/* need to finish all async __init code before freeing the memory */
@@ -1419,6 +1420,11 @@ static int __ref kernel_init(void *unused)
 
 	do_sysctl_args();
 
+	len = snprintf(str, 5, "A longer string\n");
+
+	pr_info("%s: len: %d, str: [%s]\n",
+		__func__, len, str);
+
 	if (ramdisk_execute_command) {
 		ret = run_init_process(ramdisk_execute_command);
 		if (!ret)

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette