Looked into this a little bit, and it's not a problem AFAICT. LLDB only memory maps in one place, which is from FileSpec::MemoryMapFileContents, and it passes writeable=false. LLVM uses MAP_PRIVATE when writeable==false, so from LLDB's point of view there is no functional change. The writeable codepath seems entirely unused in LLDB.
On Wed, Feb 15, 2017 at 12:58 PM Greg Clayton <gclay...@apple.com> wrote: > On Feb 15, 2017, at 11:22 AM, Zachary Turner <ztur...@google.com> wrote: > > Yea, the flag seems like one you would want to use almost always, so I'm > not opposed to having it. I'll see about making the changes in LLVM, even > if we end up not using it in LLDB, they seem useful in LLVM independently. > > BTW, one difference in LLVM's mmap code is that in LLDB we always use > MAP_PRIVATE, but in LLVM if you are opening for readwrite it uses > MAP_SHARED. Are you against using MAP_SHARED when mmaping with readwrite > privileges? > > > You will need to do some testing. If you do MAP_SHARED and the file goes > away, you might crash as it won't keep the file around as long as it needs > it. I could be wrong on this. But I do remember explicitly picking > MAP_PRIVATE for some reason in the past... > > On Wed, Feb 15, 2017 at 11:19 AM Greg Clayton <gclay...@apple.com> wrote: > > On Feb 15, 2017, at 11:07 AM, Zachary Turner <ztur...@google.com> wrote: > > If you only ever call MemoryMapContentsIfLocal, then is that first flag > even doing anything? And if you are passing that flag, then can you just > mmap it always even if non-local? > > > If that is the only call people are using, then we don't really need the > flag. Not sure how else as local file could go away such that the backing > store wouldn't be available. mmap() on unix will keep the file around as > long as its needed AFAIK, so even if someone deletes it, it would be kept > around. So if those are our only clients we should be ok. The code signing > bit would need to be added for Mac though. > > I searched the code and only in the windows minidump plugin are we > unconditionally mmaping, and that could be changed to a > conditional-on-local just like everywhere else. If we do that, then nobody > is ever mmaping any non-local file AFAICT > > > That currently is true, but it would be shame to lose the ability to be > resilient in cases where you do want to mmap. If a file is too large, we > really should probably have an upper end cutoff on file size that would > mmap even if remote. If we have a 4 GB file that we want to access, > probably not a great thing to just pop into a heap based memory buffer... > > Greg > > > On Wed, Feb 15, 2017 at 11:02 AM Greg Clayton <gclay...@apple.com> wrote: > > > On Feb 15, 2017, at 10:58 AM, Zachary Turner <ztur...@google.com> wrote: > > > On Wed, Feb 15, 2017 at 10:47 AM Reid Kleckner <r...@google.com> wrote: > > On Wed, Feb 15, 2017 at 10:35 AM, Greg Clayton via lldb-dev < > lldb-dev@lists.llvm.org> wrote: > > I am fine with switching mmap over to llvm if it works. One important > thing to LLDB is we have a "mmap if not on remote file system" that must > continue to work. If you mmap something from a network drive and then it > gets disconnected, you can crash LLDB. So we have a function we used that > implements mmap if local, read all contents if remote, that must work to > avoid crashes. > > > LLVM's MemoryBuffer API already serves too many different use cases. > Initially it was designed to be a utility for efficiently reading source > file inputs. It has a bunch of functionality around ensuring that the > buffer is null terminated, and a boolean to avoid using mmap when the user > might modify the file concurrently. It's too complicated. I wouldn't > recommend using it without a good reason beyond just reusing a platform > abstraction. mmap and MapViewOfFile are not that complicated. LLDB is > probably better off doing its own thing unless it needs to pass mapped file > contents to other parts of LLVM, like maybe clang's VFS. > > > I just took a look and it seems like almost a drop in replacement. Only > thing that would need changing is updating shouldUseMmap() to return false > if a file is on a network share. But this seems like a good change > independently of lldb. > > After that all lldb has to do is say MemoryBuffer::getOpenFile() and > everything works. > > Is there a good reason *not* to use it? Evenif internally the > implementation is complex, the external interface is not > > > The other thing is on Mac we add new flags to mmap that allow us not to > crash if the backing store (network mount) goes away. There is also a flag > that says "if code signature is borked, please still allow me to read the > file without crashing. That functionality will need to be ported into the > LLVM code somehow so we don't start crashing again. Previously we would > crash if someone would do: > > (lldb) file /tmp/invalid_codesig_app > > And also if the network mounted share would go away on something that was > mmap'ed and someone would touch any byte within it. Our version of mmap > works around this and we need that to be in any version we adopt. > > Greg > > > >
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