On Thu, Apr 20, 2017 at 9:52 PM, Kees Cook keescook@chromium.org wrote:
On Thu, Apr 20, 2017 at 3:15 AM, Arnd Bergmann arnd@arndb.de wrote:
On Sun, Apr 16, 2017 at 9:52 PM, Kees Cook keescook@chromium.org wrote:
The original gcc-4.3 release was in early 2008. If we decide to still support that, we probably want the first 10 quirks in this series, while gcc-4.6 (released in 2011) requires none of them.
I'd be in support of raising the minimum to gcc 4.6. (I'd actually prefer 4.7, just to avoid some 4.6 packaging issues, and for better gcc plugin support.)
I'm curious what gcc 4.6 binaries are common in the wild besides old-stable Debian (unsupported in maybe a year from now?) and 12.04 Ubuntu (going fully unsupported in 2 weeks). It looks like 4.6 was used only in Fedora 15 and 16 (both EOL).
I think we are better off defining two versions: One that we know a lot of people care about, and we actively try to make that work well in all configurations (e.g. 4.6, 4.7 or 4.8), fixing all warnings we run into, and an older version that we try not to break intentionally (e.g. 3.4, 4.1 or 4.3) but that we only fix when someone actually runs into a problem they can't work around by upgrading to a more modern compiler.
For "working well everywhere" I feel like 4.8 is the better of those three (I'd prefer 4.9). I think we should avoid 4.6 -- it seems not widely used.
I suspect that 4.9 might be the one that actually works best across architectures, and it contained some very significant changes. In my testing gcc-5 tends to behave very similarly to 4.9, and gcc-6 introduced a larger number of new warnings, so that would clearly be too new for a recommended version.
The suggestion of 4.9 or higher is appealing as a recommendation because it matches what I would personally tell people:
- If you have gcc-4.9 or newer and you don't rely on any newer features, there is no need to upgrade - Wth gcc-4.8, the -Wmaybe-uninitialized warnings are now turned off because they were too noisy, so upgrading is probably a good idea even though the compiler is otherwise ok and in widespread use - gcc-4.6 and 4.7 are basically usable for building kernels, but the warning output is often counterproductive, and the generated object code may be noticeably worse. - anything before gcc-4.6 is missing too many features to be useful on ARM, but may still be fine on other architectures.
On the other hand, there is a noticeable difference in compile speed, as a 5% slowdown compared to the previous release apparently is not considered a regression. These are the times I see for building ARM 'vexpress_defconfig':
gcc-4.4: real 0m47.269s user 11m48.576s gcc-4.5: real 0m44.878s user 10m58.900s gcc-4.6: real 0m44.621s user 11m34.716s gcc-4.7: real 0m47.476s user 12m42.924s gcc-4.8: real 0m48.494s user 13m19.736s gcc-4.9: real 0m50.140s user 13m44.876s gcc-5.x: real 0m51.302s user 14m05.564s gcc-6.x: real 0m54.615s user 15m06.304s gcc-7.x: real 0m56.008s user 15m44.720s
That is a factor of 1.5x in CPU cycles between slowest and fastest, so there is clearly a benefit to keeping the old versions around, but there is also no clear cut-off other thannoticing that gcc-4.4 is slower than 4.5 in this particular configuration.
For an old compiler... yikes. 3.4 sounds insane to me. :)
That was my initial thought as well. On ARM, it clearly is insane, as even gcc-4.0 is unable to build any of the modern defconfigs (lacking -mabi=aapcs, ICE when building vsprintf.c) and even the patch I did to get gcc-4.1 to build is probably too ugly to get merged, so to build any unpatched kernel after linux-3.6 you need at least gcc-4.2, or even gcc-4.4 for the ''defconfig' (gcc-4.3 if you disable vdso).
Then again, on x86, old cmpilers were claimed to be much better supported. I just tried it out and found that no x86 defconfig kernel since linux-3.2 could be built with gcc-3.4, probably not on any other architecture either (it cannot have forward declarations for inline functions and we have one in kernel/sched_fair.c).
I think that would be a really good argument for requiring something newer ;-)
The linux-4.2 x86 defconfig could still be built with gcc-4.0, but later kernels have several minor problems with that, and require at least gcc-4.3.
If we are ok with this status quo, we could simply declare gcc-4.3 the absolute minimum version for the kernel, make gcc-4.9 the recommeded minimum version, and remove all workarounds for gcc-4.2 or older.
If anyone has a good reason for gcc-4.0 through gcc-4.2, then we would need a small number of patches to get them back working with x86 defconfig.
Arnd