[ruby-core:105693] [Ruby master Bug#18255] ioctl zeroes the last buffer byte

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


`ptr` was reread just after the block, this is not a bug.

----------------------------------------
Bug #18255: ioctl zeroes the last buffer byte
https://bugs.ruby-lang.org/issues/18255#change-94192

* 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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