Fund: Kernel/LibC: Implement correct seek, read and write behavior beyond EOF

https://pubs.opengroup.org/onlinepubs/9699919799/functions/fseek.html

The fseek() function shall allow the file-position indicator to be set beyond the end of existing data in the file. If data is later written at this point, subsequent reads of data in the gap shall return bytes with the value 0 until data is actually written into the gap.

We don't support this at the moment; the lseek syscall fails when the seek offset causes the pointer to move beyond the EOF.

First we need to decide what makes sense in kernel vs. userspace: do we want to implement this behavior all the way into the lseek syscall, or can we deal with out-of-bounds shenanigans in userspace?

Current behavior:

serenity/Kernel/FileSystem/FileDescription.cpp

Lines 119 to 156 in 8f81d9a

    
           KResultOr<off_t> FileDescription::seek(off_t offset, int whence) 
        
           { 
        
               Locker locker(m_lock); 
        
               if (!m_file->is_seekable()) 
        
                   return ESPIPE; 
        
               off_t new_offset; 
        
               switch (whence) { 
        
               case SEEK_SET: 
        
                   new_offset = offset; 
        
                   break; 
        
               case SEEK_CUR: 
        
                   if (Checked<off_t>::addition_would_overflow(m_current_offset, offset)) 
        
                       return EOVERFLOW; 
        
                   new_offset = m_current_offset + offset; 
        
                   break; 
        
               case SEEK_END: 
        
                   if (!metadata().is_valid()) 
        
                       return EIO; 
        
                   new_offset = metadata().size; 
        
                   break; 
        
               default: 
        
                   return EINVAL; 
        
               } 
        
               if (new_offset < 0) 
        
                   return EINVAL; 
        
               // FIXME: Return EINVAL if attempting to seek past the end of a seekable device. 
        
               m_current_offset = new_offset; 
        
               m_file->did_seek(*this, new_offset); 
        
               if (m_inode) 
        
                   m_inode->did_seek(*this, new_offset); 
        
               evaluate_block_conditions(); 
        
               return m_current_offset; 
        
           }

SerenityOS/serenity

Kernel/LibC: Implement correct seek, read and write behavior beyond EOF

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	KResultOr<off_t> FileDescription::seek(off_t offset, int whence)
	{
	Locker locker(m_lock);
	if (!m_file->is_seekable())
	return ESPIPE;

	off_t new_offset;

	switch (whence) {
	case SEEK_SET:
	new_offset = offset;
	break;
	case SEEK_CUR:
	if (Checked<off_t>::addition_would_overflow(m_current_offset, offset))
	return EOVERFLOW;
	new_offset = m_current_offset + offset;
	break;
	case SEEK_END:
	if (!metadata().is_valid())
	return EIO;
	new_offset = metadata().size;
	break;
	default:
	return EINVAL;
	}

	if (new_offset < 0)
	return EINVAL;
	// FIXME: Return EINVAL if attempting to seek past the end of a seekable device.

	m_current_offset = new_offset;

	m_file->did_seek(*this, new_offset);
	if (m_inode)
	m_inode->did_seek(*this, new_offset);
	evaluate_block_conditions();
	return m_current_offset;
	}

SerenityOS/serenity

Kernel/LibC: Implement correct seek, read and write behavior beyond EOF

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