[ruby-core:57110] [ruby-trunk - Bug #8864] sprintf segfaults with too high precision

Issue #8864 has been updated by utkarshkukreti (Utkarsh Kukreti).


I'm trying to write a patch for this (my first contribution actually), and I'll really appreciate some help.

I've found the cause -- the buffer sent to `cvt()` function in vsnprintf.c is allocated on the stack with a fixed size of `#define BUF (MAXEXP+MAXFRACT+1)` [here](
https://github.com/ruby/ruby/blob/5b46f6c602c24c9cdf995914fc6998981f1e53ec/vsnprintf.c#L502) which on my machine is `1024 + 64 + 1 == 1089`, and the data is written to it without any bounds check, which causes the segfault.

I can think of two possible solutions:

1. Limit the precision a user can specify on a call to sprintf to `MAXFRACT`.
2. `malloc` the actual required memory when it's greater than the defined constant `BUF`, and `free` it before returning from the function.

I think (2) is the best solution here.

What do you all think? Also, what functions should I use to allocate/free memory inside `vsnprintf`?
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Bug #8864: sprintf segfaults with too high precision
https://bugs.ruby-lang.org/issues/8864#change-41720

Author: Aaronneyer (Aaron Neyer)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 
ruby -v: 
Backport: 


In any Ruby version (attempted with 1.8.7, 1.9.3, and 2.0.0), specifying a very large precision in sprintf can cause a segmentation fault.

The following code will cause the segmentation fault.

`"%.99999f" % 10`

The number to cause a segfault is dependent on the system. On my laptop, any number above 1100 would cause it, and on an EC2 micro instance, around 2500 was the limit.


-- 
http://bugs.ruby-lang.org/