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Double Faults: A missing handler leads to a #GP exception (not a #NP)

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A IDT entry that is zero is not valid (e.g. the must-be-one bits are not set). For this reason, a general protection fault occurs instead of a segment-not-present fault.

See https://github.com/phil-opp/blog_os/issues/449#issuecomment-558906316 for more information.
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Philipp Oppermann
2020-02-19 10:58:55 +01:00
parent a392065ca8
commit b532c052ad
1 changed files with 4 additions and 4 deletions
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8
blog/content/second-edition/posts/06-double-faults/index.md
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@@ -58,8 +58,8 @@ We use `unsafe` to write to the invalid address `0xdeadbeef`. The virtual addres
When we start our kernel now, we see that it enters an endless boot loop. The reason for the boot loop is the following: When we start our kernel now, we see that it enters an endless boot loop. The reason for the boot loop is the following:
1. The CPU tries to write to `0xdeadbeef`, which causes a page fault. 1. The CPU tries to write to `0xdeadbeef`, which causes a page fault.
2. The CPU looks at the corresponding entry in the IDT and sees that the present bit isn't set. Thus, it can't call the page fault handler and a double fault occurs. 2. The CPU looks at the corresponding entry in the IDT and sees that no handler function is specified. Thus, it can't call the page fault handler and a double fault occurs.
3. The CPU looks at the IDT entry of the double fault handler, but this entry is also non-present. Thus, a _triple_ fault occurs. 3. The CPU looks at the IDT entry of the double fault handler, but this entry does not specify a handler function either. Thus, a _triple_ fault occurs.
4. A triple fault is fatal. QEMU reacts to it like most real hardware and issues a system reset. 4. A triple fault is fatal. QEMU reacts to it like most real hardware and issues a system reset.
So in order to prevent this triple fault, we need to either provide a handler function for page faults or a double fault handler. We want to avoid triple faults in all cases, so let's start with a double fault handler that is invoked for all unhandled exception types. So in order to prevent this triple fault, we need to either provide a handler function for page faults or a double fault handler. We want to avoid triple faults in all cases, so let's start with a double fault handler that is invoked for all unhandled exception types.
@@ -98,7 +98,7 @@ When we start our kernel now, we should see that the double fault handler is inv
It worked! Here is what happens this time: It worked! Here is what happens this time:
1. The CPU executes tries to write to `0xdeadbeef`, which causes a page fault. 1. The CPU executes tries to write to `0xdeadbeef`, which causes a page fault.
2. Like before, the CPU looks at the corresponding entry in the IDT and sees that the present bit isn't set. Thus, a double fault occurs. 2. Like before, the CPU looks at the corresponding entry in the IDT and sees that no handler function is defined. Thus, a double fault occurs.
3. The CPU jumps to the – now present – double fault handler. 3. The CPU jumps to the – now present – double fault handler.
The triple fault (and the boot-loop) no longer occurs, since the CPU can now call the double fault handler. The triple fault (and the boot-loop) no longer occurs, since the CPU can now call the double fault handler.
@@ -146,7 +146,7 @@ With the help of this table, we can answer the first three of the above question
2. If a page fault occurs and the page fault handler is swapped out, a _double fault_ occurs and the _double fault handler_ is invoked. 2. If a page fault occurs and the page fault handler is swapped out, a _double fault_ occurs and the _double fault handler_ is invoked.
3. If a divide-by-zero handler causes a breakpoint exception, the CPU tries to invoke the breakpoint handler. If the breakpoint handler is swapped out, a _page fault_ occurs and the _page fault handler_ is invoked. 3. If a divide-by-zero handler causes a breakpoint exception, the CPU tries to invoke the breakpoint handler. If the breakpoint handler is swapped out, a _page fault_ occurs and the _page fault handler_ is invoked.
In fact, even the case of a non-present handler follows this scheme: A non-present handler causes a _segment-not-present_ exception. We didn't define a segment-not-present handler, so another segment-not-present exception occurs. According to the table, this leads to a double fault. In fact, even the case of an exception without a handler function in the IDT follows this scheme: When the exception occurs, the CPU tries to read the corresponding IDT entry. Since the entry is 0, which is not a valid IDT entry, a _general protection fault_ occurs. We did not define a handler function for the general protection fault either, so another general protection fault occurs. According to the table, this leads to a double fault.
### Kernel Stack Overflow ### Kernel Stack Overflow
Let's look at the fourth question: Let's look at the fourth question: