[Toybox] [PATCH 1/2] Handle large read and write lengths.

Rob Landley rob at landley.net
Mon Aug 9 00:44:45 PDT 2021

Sorry for the delay, I have a _really_ bad cold.

On 8/7/21 7:11 AM, Samanta Navarro wrote:
> The functions readall and writeall can return an error value by mistake
> if more than 2 GB of data are read or written.

That was intentional. If your file sizes are that big we probably want to mmap()
stuff. Single atomic transactions greater than 2 gigabytes are probably a bad
idea from a latency standpoint (circa 2015 kernel used to hold locks across
these, probably fixed now but I haven't checked) and mallocing buffers that big
is probably also a bad idea. (I hit a glibc version that wouldn't allow a malloc
greater than 128 megs, haven't tested recently...)

That said, I'd want it to hard error rather than integer overflow back into a
sane value (6 gigs being treated as 2...)

> This happens because int and ssize_t are of different sizes on 64 bit
> architectures. Using ssize_t resolves the issue because read and write
> return ssize_t already.

Actually on 32 bit Linux architectures ssize_t is also long long because "large
file support" was introduced over 20 years ago:


And 5 years later even 2 terabytes was limiting:


So if we're changing the type it should change to long long, but this is really
pilot error in the callers. (That said, readfd() and lskip() are making this
mistake, so there needs to _be_ an enforcement mechanism...)


P.S. One of my most unsolvable todo items is what to do about readline() on
/dev/zero. If it's looking for /n it's just gonna allocate a bigger and bigger
buffer until it triggers the OOM killer. If a single line IS a gigabyte long,
what am I supposed to _do_ about it?

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