https://bugs.openldap.org/show_bug.cgi?id=9397
Issue ID: 9397
Summary: LMDB: A second process opening a file with
MDB_WRITEMAP can cause the first to SIGBUS
Product: LMDB
Version: 0.9.26
Hardware: All
OS: All
Status: UNCONFIRMED
Severity: normal
Priority: ---
Component: liblmdb
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
Created attachment 780
--> https://bugs.openldap.org/attachment.cgi?id=780&action=edit
Full reproduction of SIGBUS MDB_WRITEMAP issue (works on Linux only)
The fundamental problem is that a ftruncate() on Linux that makes a file
smaller will cause accesses past the new end of the file to SIGBUS (see the
mmap man page).
The sequence that causes a SIGBUS involves two processes.
1. The first process opens a new LMDB file with MDB_WRITEMAP.
2. The second process opens the same LMDB file with MDB_WRITEMAP and with an
explicit map_size smaller than the first process's map size.
* This causes an ftruncate that makes the underlying file *smaller*.
3. (Optional) The second process closes the environment and exits.
4. The first process opens a write transaction and writes a bunch of data.
5. The first process commits the transaction. This causes a memory read from
the mapped memory that's now past the end of the file. On Linux, this triggers
a SIGBUS.
Attached is code that fully reproduces the problem on Linux.
The most straightforward solution is to allow ftruncate to *reduce* the file
size if it is the only reader. Another possibility is check the file size and
ftruncate if necessary every time a write transaction is opened. A third
possibility is to catch the SIGBUS signal.
Repro note: I used clone() to create the subprocess to most straightforwardly
demonstrate that the problem is not due to inherited file descriptors. The
problem still manifests when the processes are completely independent.
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