From: Jeff Hostetler <git@jeffhostetler.com>
Date: Mon, 25 Jan 2021 19:34:27 GMT
Subject: Re: [PATCH 01/10] pkt-line: use stack rather than static buffer in packet_write_gently()
Message-ID: <04f62d1a-42bb-cc45-0e05-a00bfd5eceba@jeffhostetler.com>
In-Reply-To: <X/71qByO5jSceIFn@coredump.intra.peff.net>



On 1/13/21 8:29 AM, Jeff King wrote:
> On Tue, Jan 12, 2021 at 03:31:23PM +0000, Jeff Hostetler via GitGitGadget wrote:
> 
>> Teach packet_write_gently() to use a stack buffer rather than a static
>> buffer when composing the packet line message.  This helps get us ready
>> for threaded operations.
> 
> Sounds like a good goal, but...
> 
>>   static int packet_write_gently(const int fd_out, const char *buf, size_t size)
>>   {
>> -	static char packet_write_buffer[LARGE_PACKET_MAX];
>> +	char packet_write_buffer[LARGE_PACKET_MAX];
>>   	size_t packet_size;
> 
> 64k is awfully big for the stack, especially if you are thinking about
> having threads. I know we've run into issues around that size before
> (though I don't offhand recall whether there was any recursion
> involved).
> 
> We might need to use thread-local storage here. Heap would also
> obviously work, but I don't think we'd want a new allocation per write
> (or maybe it wouldn't matter; we're making a syscall, so a malloc() may
> not be that big a deal in terms of performance).
> 
> -Peff
> 

Good point.

I'll look at the callers and see if I can do something safer.

Jeeff