On Fri, Aug 13, 2021, at 5:31 PM, Linus Torvalds wrote: > On Fri, Aug 13, 2021 at 10:18 AM Eric W. Biederman > <ebiederm@xxxxxxxxxxxx> wrote: > > > > Florian Weimer, would it be possible to get glibc's ld.so implementation to use > > MAP_SHARED? Just so people reading the code know what to expect of the > > kernel? As far as I can tell there is not a practical difference > > between a read-only MAP_PRIVATE and a read-only MAP_SHARED. > > There's a huge difference. > > For one, you actually don't necessarily want read-only. Doing COW on > library images is quite common for things like relocation etc (you'd > _hope_ everything is PC-relative, but no) > > So no. Never EVER use MAP_SHARED unless you literally expect to have > two different mappings that need to be kept in sync and one writes the > other. > > I'll just repeat: stop arguing about this case. If somebody writes to > a busy library, THAT IS A FUNDAMENTAL BUG, and nobody sane should care > at all about it apart from the "you get what you deserve". > > What's next? Do you think glibc should also map every byte in the user > address space so that user programs don't get SIGSEGV when they have > wild pointers? > > Again - that's a user BUG and trying to "work around" a wild pointer > is a worse fix than the problem it tries to fix. > > The exact same thing is true for shared library (or executable) > mappings. Trying to work around people writing to them is *worse* than > the bug of doing so. > > Stop this completely inane discussion already. > I’ll bite. How about we attack this in the opposite direction: remove the deny write mechanism entirely. In my life, I’ve encountered -ETXTBUSY intermittently, and it invariably means that I somehow failed to finish killing a program fast enough for whatever random rebuild I’m doing to succeed. It’s at best erratic — it only applies for static binaries, and it has never once saved me from a problem I care about. If the program I’m recompiling crashes, I don’t care — it’s probably already part way through dying from an unrelated fatal signal. What actually happens is that I see -ETXTBUSY, think “wait, this isn’t Windows, why are there file sharing rules,” then think “wait, Linux has *one* half baked file sharing rule,” and go on with my life. [0] Seriously, can we deprecate and remove the whole thing? [0] we have mandatory locks, too. Sigh.
