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c67a91f11a
WITH MYISAM_USE_MMAP ENABLED
MySQL server can crash due to segmentation fault when
started with myisam_use_mmap.
The reason behind this being, while making a request to
unmap (munmap) the previously mapped memory (mmap), the
size passed was 7 bytes larger than the size requested at
the time of mapping. This can eventually unmap the adjacent
memory mapped block, belonging to some other memory-map pool.
Hence the subsequent call to mmap can map a region which was
still a valid memory mapped area.
Fixed by removing the extra 7-byte margin which was erroneously
added to the size, used for unmappping.
storage/myisam/mi_close.c:
Bug#11756764 48726: MYSQLD KEEPS CRASHING WITH SIGSEGV
WITH MYISAM_USE_MMAP ENABLED
Added a condition to call _mi_unmap_file() in case
of compressed records. mi_munmap_file() is called
otherwise.
storage/myisam/mi_packrec.c:
Bug#11756764 48726: MYSQLD KEEPS CRASHING WITH SIGSEGV
WITH MYISAM_USE_MMAP ENABLED
mi_dynmap_file() function, after successfully executing
mmap, stores the total size in info->s->mapped_length
variable. Now, if mi_dynmap_file() is invoked with a size
with an extra 7-byte margin (MEMMAP_EXTRA_MARGIN),
the margin will eventually also get stored in mapped_length.
So, un-mapping function can simply use the value stored in
mapped_length in order to unmap the previously mapped
region.