[PATCH] Set behavior of sprintf-like functions with overlapping source and destination

[Siddhesh Poyarekar]

On 27/12/18 3:52 AM, Paul Eggert wrote:
>> I wonder if something like the following would be too complex:
>>
>>   1. Introduce GLIBC_2.29 printf that does *not* support this undefined
>>      behavior.  Use __asm__(".symver [...]@@GLIBC_2.0") to ensure we
>>      still default to the GLIBC_2.0 version even when building new
>>      programs.
>>
>>   2. After a critical mass of users are using glibc 2.29 or newer (e.g.
>>      after a year), switch the default version to GLIBC_2.29.
>>
>> What do you think?
> 
> I'm afraid it sounds confusing, as GLIBC_2.29 wouldn't be the default 
> for glibc 2.29 when it's released. Perhaps the symbol should be 
> GLIBC_UNSTABLE instead? But even then, I don't see why users would use 
> the new version before the default was switched, so if there are issues 
> we won't find them any more gently with this approach.

We should not retain compatible behaviour for building, only for linking 
like we normally do.  The symbol versioning patch won't get backported 
(since it is an ABI event) to older distributions and as long as it is 
alongside gcc8+ (I tested 8, the warning might have been introduced 
earlier) there is an adequate warning that tells users their houses may 
burn down if they depend on this behaviour.  Or something like that.

The risk is not completely gone though and there may be environments 
(gcc5 for example) where there is no warning.  These would be custom 
built environments though and I suppose we can depend on these devs to 
read the manpage more carefully.  Or maybe add more code in the headers 
to link against the compat *printf if an older compiler is detected? 
It's debatable if all of this is necessary for undefined behaviour...

In any case, this doesn't look like something that'll be done within 
this week, so I propose we add this compatibility change in now (without 
the UB test) in the interest of not breaking things and then add 
versioning to only retain backward compatible behaviour for GLIBC_2.0 
printf after discussing at length how much breakage we can stand.  Maybe 
also get friends at LWN to write about how sprintf using aliased buffers 
is asking for trouble.

With this action plan we don't really need to test the undefined 
behaviour since we don't intend to retain it except in the compat case, 
just flip the switch in the ub-chk test later so that it always does the 
check instead of only under _FORTIFY_SOURCE.

Siddhesh