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iSCSI causes memory corruption
Description of problem:
This is a compound problem, which most likely involves a combination of how TrueNAS SCALE handles iSCSI triggering a problem **and** some memory-handling issue in QEMU leading to a crash. In short any Linux machine started with iSCSI handled by QEMU directly leads to a hard crash within 30s-1h. I was able to find a pattern in logs:
1. First, a message like `QEMU[53139]: kvm: iSCSI Busy/TaskSetFull/TimeOut (retry #1 in 0 ms): TASK_SET_FULL` is logged
- it is always `TASK_SET_FULL`
- it is always `retry #1 in ... ms`, where only number of miliseconds varies
- the line is repeated multiple times, sometimes 5x and sometimes >200x
2. It is followed by a single line with one of the following:
- `double free or corruption (out)`
- `double free or corruption (!prev)`
- `kvm: ../block/block-backend.c:1567: blk_aio_write_entry: Assertion `!qiov || qiov->size == acb->bytes' failed.`
- `kvm: malloc.c:2379: sysmalloc: Assertion `(old_top == initial_top (av) && old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse (old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)' failed.`
- `kvm: iSCSI CheckCondition: SENSE KEY:UNIT_ATTENTION(6) ASCQ:BUS_RESET(0x2900)`
- `malloc(): invalid size (unsorted)`
3. The virtual machine crashes
Steps to reproduce:
I don't have a specific concrete steps, only clues really. This problem started happening after TrueNAS SCALE updated their iSCSI code in Bluefin release to a new upstream version. That iSCSI server still works when iSCSI is mounted by the kernel and QEMU uses a normal `/dev` entry. While there's probably some problem with it, QEMU shouldn't probably crash with memory errors.
Additional information:
While I'm a software developer, I don't code in C on a daily basis. However, looking at the errors, I have a suspicion the problem may be somewhere in the `iscsi_co_generic_cb()`, as it seems the struct is getting damaged (out of bound write?) and causes explosion somewhere down the line.
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