[PATCH] greybus: remove stray nul byte in apb_log_enable_read output

[PATCH] greybus: remove stray nul byte in apb_log_enable_read output

[Rasmus Villemoes]

Including a nul byte in the otherwise human-readable ascii output
from this debugfs file is probably not intended.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 drivers/greybus/es2.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/greybus/es2.c b/drivers/greybus/es2.c
index 48ad154df3a7..86a7fbc7fe13 100644
--- a/drivers/greybus/es2.c
+++ b/drivers/greybus/es2.c
@@ -1171,7 +1171,7 @@ static ssize_t apb_log_enable_read(struct file *f, char __user *buf,
 	char tmp_buf[3];
 
 	sprintf(tmp_buf, "%d\n", enable);
-	return simple_read_from_buffer(buf, count, ppos, tmp_buf, 3);
+	return simple_read_from_buffer(buf, count, ppos, tmp_buf, 2);
 }
 
 static ssize_t apb_log_enable_write(struct file *f, const char __user *buf,
-- 
2.29.2

Re: [greybus-dev] [PATCH] greybus: remove stray nul byte in apb_log_enable_read output

[Alex Elder]

On 3/26/21 10:22 AM, Rasmus Villemoes wrote:
> Including a nul byte in the otherwise human-readable ascii output
> from this debugfs file is probably not intended.

Looking only at the comments above simple_read_from_buffer(),
the last argument is the size of the buffer (tmp_buf in this
case).  So "3" is proper.

But looking at callers, it seems that the trailing NUL is
often excluded this way.

I don't really have a problem with your patch, but could
you explain why having the NUL byte included is an actual
problem?  A short statement about that would provide better
context than just "probably not intended."

Thanks.

					-Alex
> 
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
>   drivers/greybus/es2.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/greybus/es2.c b/drivers/greybus/es2.c
> index 48ad154df3a7..86a7fbc7fe13 100644
> --- a/drivers/greybus/es2.c
> +++ b/drivers/greybus/es2.c
> @@ -1171,7 +1171,7 @@ static ssize_t apb_log_enable_read(struct file *f, char __user *buf,
>   	char tmp_buf[3];
>   
>   	sprintf(tmp_buf, "%d\n", enable);
> -	return simple_read_from_buffer(buf, count, ppos, tmp_buf, 3);
> +	return simple_read_from_buffer(buf, count, ppos, tmp_buf, 2);
>   }
>   
>   static ssize_t apb_log_enable_write(struct file *f, const char __user *buf,
> 

Re: [greybus-dev] [PATCH] greybus: remove stray nul byte in apb_log_enable_read output

[Rasmus Villemoes]

On 26/03/2021 17.31, Alex Elder wrote:
> On 3/26/21 10:22 AM, Rasmus Villemoes wrote:
>> Including a nul byte in the otherwise human-readable ascii output
>> from this debugfs file is probably not intended.
> 
> Looking only at the comments above simple_read_from_buffer(),
> the last argument is the size of the buffer (tmp_buf in this
> case).  So "3" is proper.

The size of the buffer is 3 because that's what sprintf() is gonna need
to print one digit, '\n' and a nul byte. That doesn't necessarily imply
that the entire buffer is meant to be sent to userspace.

> But looking at callers, it seems that the trailing NUL is
> often excluded this way.
> 
> I don't really have a problem with your patch, but could
> you explain why having the NUL byte included is an actual
> problem?  A short statement about that would provide better
> context than just "probably not intended."

debugfs files are AFAIK intended to be used with simple "cat foo", "echo
1 > foo" in shell (scripts and interactive). Having non-printable
characters returned from that "cat foo" is odd (and can sometimes break
scripts that e.g. "grep 1 foo/*/*/bar" when grep stops because it thinks
one of the files is binary, or when the output of that is further piped
somewhere).

At the very least, it's inconsistent for this one, those in
greybus/svc.c do just return the ascii digits and the newline (and if
one followed your argument above and let those pass 16 instead of desc
one would leak a few bytes of uninitialized kernel stack to userspace).

I said "probably not intended" because for all I know, it might be
intentional. I just doubt it.

Rasmus

Re: [greybus-dev] [PATCH] greybus: remove stray nul byte in apb_log_enable_read output

[Alex Elder]

On 3/26/21 12:05 PM, Rasmus Villemoes wrote:
> On 26/03/2021 17.31, Alex Elder wrote:
>> On 3/26/21 10:22 AM, Rasmus Villemoes wrote:
>>> Including a nul byte in the otherwise human-readable ascii output
>>> from this debugfs file is probably not intended.
>>
>> Looking only at the comments above simple_read_from_buffer(),
>> the last argument is the size of the buffer (tmp_buf in this
>> case).  So "3" is proper.
> 
> The size of the buffer is 3 because that's what sprintf() is gonna need
> to print one digit, '\n' and a nul byte. That doesn't necessarily imply
> that the entire buffer is meant to be sent to userspace.
> 
>> But looking at callers, it seems that the trailing NUL is
>> often excluded this way.
>>
>> I don't really have a problem with your patch, but could
>> you explain why having the NUL byte included is an actual
>> problem?  A short statement about that would provide better
>> context than just "probably not intended."

My point was really that you should have provided a better
explanation in your description.

At this point it's been discussed enough so I won't ask you
to post version 2.

Acked-by: Alex Elder <elder@linaro.org>

> 
> debugfs files are AFAIK intended to be used with simple "cat foo", "echo
> 1 > foo" in shell (scripts and interactive). Having non-printable
> characters returned from that "cat foo" is odd (and can sometimes break
> scripts that e.g. "grep 1 foo/*/*/bar" when grep stops because it thinks
> one of the files is binary, or when the output of that is further piped
> somewhere).
> 
> At the very least, it's inconsistent for this one, those in
> greybus/svc.c do just return the ascii digits and the newline (and if
> one followed your argument above and let those pass 16 instead of desc
> one would leak a few bytes of uninitialized kernel stack to userspace).
> 
> I said "probably not intended" because for all I know, it might be
> intentional. I just doubt it.
> 
> Rasmus
>