Issue #18255 has been updated by vihai (Daniele Orlandi).


nobu (Nobuyoshi Nakada) wrote in #note-3:
> Found the bug.
> Does this patch fix it?

Here I am.

Apparently it doesn't.
I dug a bit deeper and I found that there are two issues that concur to this behavior:

* `ioctl_narg_len` isn't properly extracting the buffer size from the ioctl number and defaults to `DEFULT_IOCTL_NARG_LEN`
* When the original buffer is bigger than `DEFULT_IOCTL_NARG_LEN` it is not expanded to make room to the sentinel byte which is then overwrote with the string terminator.

The first issue is caused by `<sys/ioctl.h>` not defining `_IOC_SIZE`, ruby falls back to `DEFULT_IOCTL_NARG_LEN`. I guess you have to detect and include `<linux/ioctl.h>` or `<asm/ioctl.h>`.

The second may be patched like this:

```diff
--- io.c
+++ io.c.patched
@@ -9823,7 +9823,11 @@
                rb_str_resize(arg, len+1);
                MEMZERO(RSTRING_PTR(arg)+slen, char, len-slen);
                slen = len+1;
+           } else {
+               rb_str_resize(arg, slen+1);
+               slen++;
            }
+
            /* a little sanity check here */
            ptr = RSTRING_PTR(arg);
            ptr[slen - 1] = 17;
```

Lastly I guess that DEFULT is spelled incorrectly :)

----------------------------------------
Bug #18255: ioctl zeroes the last buffer byte
https://bugs.ruby-lang.org/issues/18255#change-94233

* Author: vihai (Daniele Orlandi)
* Status: Open
* Priority: Normal
* Backport: 2.6: REQUIRED, 2.7: REQUIRED, 3.0: REQUIRED
----------------------------------------

Hello,

I'm running ruby 2.7.4p191 on an armv7 linux and experimenting with GPIO_GET_LINEHANDLE_IOCTL ioctl.

The ioctl sanity check is triggered as if the buffer was too small however the size of the buffer passed to ioctl is correct.

```
io.rb:116:in `ioctl': return value overflowed string (ArgumentError)
```

If I append at least one byte to the buffer the ioctl does not raise an exception.

It seems that the last byte of the buffer is zeroed:


```
puts "SIZE=#{req.bytesize}"
req = req + "XXXXXXXXXX".b     
puts req.unpack("H*")       
fd.ioctl(GPIO_GET_LINEHANDLE_IOCTL, req)                        
puts req.unpack("H*")     
```

```
SIZE=364
[...]0000000000000058585858585858585858
[...]0000000600000058585858585858585800
```

I checked with a C program and the ioctl does not actually touch the buffer beyond the expected 364 bytes.
The ioctl number does encode 364 as size:

```
#include <stdio.h>
#include <linux/gpio.h>

void main()
{
  printf("SIZE=%d", _IOC_SIZE(GPIO_GET_LINEHANDLE_IOCTL));
}
```

```
SIZE=364
```




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