Issue #18255 has been updated by nobu (Nobuyoshi Nakada).

Backport changed from 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN to 2.6: REQUIRED, 2.7: REQUIRED, 3.0: REQUIRED
Status changed from Feedback to Open

Found the bug.
Does this patch fix it?

```diff
diff --git a/io.c b/io.c
index 50c9fea62c9..052155205d6 100644
--- a/io.c
+++ b/io.c
@@ -10143,8 +10143,8 @@ setup_narg(ioctl_req_t cmd, VALUE *argp, int io_p)
 	    /* expand for data + sentinel. */
 	    if (slen < len+1) {
 		rb_str_resize(arg, len+1);
-		MEMZERO(RSTRING_PTR(arg)+slen, char, len-slen);
-		slen = len+1;
+		RSTRING_GETMEM(arg, ptr, slen);
+		MEMZERO(ptr+slen, char, len-slen);
 	    }
 	    /* a little sanity check here */
 	    ptr = RSTRING_PTR(arg);
```

----------------------------------------
Bug #18255: ioctl zeroes the last buffer byte
https://bugs.ruby-lang.org/issues/18255#change-94181

* 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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