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a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=kj9zAlcOel0A:10 a=dt9VzEwgFbYA:10 a=GoEa3M9JfhUA:10 a=j3hNiaGS3yfY-TIBGyoA:9 a=CjuIK1q_8ugA:10 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwNTIwMDEwOSBTYWx0ZWRfX7gQBXRTp2UL0 p2igSszB30b7Ie0ERte3/0Xx7Ruu/PaUdrpcS1ktEgP+VhRFqVjkfcuelXJVeuwrqnDl9ZEOGUO RbmalsUbX2uR36iZ6Xqu+9xgT0666/cnAfiWCZc3g1AiGhP94JflpjgxxJXb/xs49pAPWlUhvTh hSBD3WBNkj/h/LqcJWkuCDPBvC4cdZM+E9OwsyaC2rrM/M74uTKOLO0lq40FxQ1nEETtCZvg8tN UcotJW+/sV3BDT9IoXAZxUlKhr/XEq3Js8HArGfMbbvidglWkDrRnDTHcNzjIga2LSGzlqZI4Kq 0Xj6cwk2UzLnz2PgMEXffga8qJKEzCTcPkBX8YH0ebshRd5I4bAxLSA6a0Gzne+u0/j6mtXHSyq SCKerHZEcnGxOi9R7sh5DkRrbSykIuN6D6QMHP0WciUWRHqYU+JGpOwayEU11hPkyRlLTUy8 X-Proofpoint-GUID: 9ORHkRFl0WXEnUxo09GdwBfKq3Vfsvp1 On Tue, May 20, 2025 at 12:41:00PM +0100, Pedro Falcato wrote: > On Tue, May 20, 2025 at 11:21:33AM +0100, Lorenzo Stoakes wrote: > > On Tue, May 20, 2025 at 09:38:50AM +0100, Pedro Falcato wrote: > > > On Mon, May 19, 2025 at 09:52:41PM +0100, Lorenzo Stoakes wrote: > > > > It's useful in certain cases to be able to default-enable an madvise() flag > > > > for all newly mapped VMAs, and for that to survive fork/exec. > > > > > > > > The natural place to specify something like this is in an madvise() > > > > invocation, and thus providing this functionality as a flag to > > > > process_madvise() makes sense. > > > > > > > > We intentionally limit this only to flags that we know should function > > > > correctly without issue, and to be conservative about this, so we initially > > > > limit ourselves only to MADV_HUGEPAGE, MADV_NOHUGEPAGE, that is - setting > > > > the VM_HUGEPAGE, VM_NOHUGEPAGE VMA flags. > > > > > > > > We implement this functionality by using the mm_struct->def_flags field. > > > > > > This seems super specific. How about this: > > > > > > - PMADV_FUTURE (mirrors MCL_FUTURE). This only applies the flag to future VMAs in the current process. > > > - PMADV_INHERIT_FORK. This makes it so the flag is propagated to child processes (does not imply PMADV_FUTURE) > > > - PMADV_INHERIT_EXEC. This makes it so the flag is propagated through the execve boundary > > > (and this is where we'd filter for 'safe' flags, at least through the secureexec boundary). Does not imply > > > FUTURE nor INHERIT_FORK. > > > > I don't know how we could implement separate current process, fork, exec, fork/exec. > > mm->def_flags is propagated this way automatically. > > > > And again on the security stuff, I think the correct answer is to require sys > > admin capability to be able to use this option _at all_. This simplifies > > everything. > > > > To have this kind of thing we'd have to add a whole new mechanism, literally > > just for this, and I'd really rather not generate brand new mm_struct flags for > > every possible mode (in fact that would probably makes the whole thing > > intractible), or add a new field there for this. > > > > The idea is that we get the advantages of an improved madvise interface, while > > also providing the interface Usama wants without having to add some hideous > > prctl() whose logic is disconnected from the rest of madvise(), while being, in > > effect, a 'default madvise() for new mappings'. > > > > So while specific to the case, nothing prevents us in future adding more > > functionality if we want. > > > > We could also potentially: > > > > - add PMADV_SET_DEFAULT (I'm iffy about PMADV_FUTURE... but whichever we go with) > > - add PMADV_INHERIT_FORK > > - add PMADV_INHERIT_EXEC > > > > And only support PMADV_SET_DEFAULT | PMADV_INHERIT_FORK | PMADV_INHERIT_EXEC for > > now. > > > > THen we could have the security semantics you specify (require cap sys admin on > > PMADV_INHERIT_EXEC) but have that propagate to the only supported case. > > > > What do you think? > > > > If you don't want to add new fields, this option seems fine. > And then if any other usecase pops up, we're ready. Yeah sounds fair, will do on respin! > > > > > > > and, while we're at it, rename PMADV_ENTIRE_ADDRESS_SPACE to PMADV_CURRENT, to align it with MCL_CURRENT. > > > > I'm not sure making the mlock()/madvise() stuff analagous is a good idea, as > > they have different semantics. I'd rather keep these flags descriptive. Though > > I'm open to alternative naming of course... > > Semantics are similar I think? And I do think getting shorter names is a good > idea, however I won't insist too hard on this. Yeah perhaps with _ALL_ thrown in to make this clear... :) warming to it... ;) > > -- > Pedro