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BUG: Segmentation fault when -object memory-backend-file use readonly=on, prealloc=on together
Description of problem:
Segmentation Fault while booting VM.
Steps to reproduce:
1. set qemu boot params to `-object memory-backend-file,id=mem1,readonly=on,prealloc=on,mem-path=<any-img-file>,size=4G`
2.
3.
Additional information:
It might not be a bug, probably a feature.
The reason of this segfault is:
readonly would mmap the backend file using PROT_READ, make it readonly,
but the prealloc=on would touch_pages the memory mmaped by the file.
SO the segfault happens.
But there is no docs about this segfault condition (the readonly and prealloc cannot be used together.)
And maybe there is a way to solve this problem, I think.
Use mmap the memory backend file to PROT_READ|PROT_WRITE at the beginnning, after touch_pages, then mprotect the memory.
change the prot to readonly if required.
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