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Documentation/Kernel: Explain why and when to use FixedCharBuffers

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Liav A 2023-07-21 09:30:25 +03:00 committed by Andrew Kaster
parent 58b509584a
commit 1f88109d78
Notes: sideshowbarker 2024-07-16 18:03:21 +09:00 
Author: https://github.com/supercomputer7
Commit: https://github.com/SerenityOS/serenity/commit/1f88109d78
Pull-request: https://github.com/SerenityOS/serenity/pull/20037
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/kleinesfilmroellchen ✅
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Documentation/Kernel/DevelopmentGuidelines.md
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@ -36,6 +36,33 @@ but because of the async operation, we can't send the `errno` code back to userl
to ensure that internal functions still use the `ErrorOr<>` return type, and in main calling function, we use to ensure that internal functions still use the `ErrorOr<>` return type, and in main calling function, we use
other meaningful infrastructure utilities in the Kernel to indicate that the operation failed. other meaningful infrastructure utilities in the Kernel to indicate that the operation failed.
## KStrings vs FixedStringBuffers
As you might understand, we put a respectable amount of effort into making the kernel code OOM-safe.
One approach to achieve this is to allow error propagation where possible.
The other approach is to eliminate heap allocations altogether where possible.
To do so, the FixedStringBuffer class was introduced into the AK library, and is used
extensively in kernel syscall handlers' code.
The idea is very simple - if we know the maximum length of an inspected string during
a syscall and it's relatively short (so it doesn't exceed the stack size), something like
1024 bytes is the total max length (but in theory we could just make the stack size bigger),
it could be copied from userspace to that stack storage instead of doing an heap allocation
to create a KString. This is especially useful when inspecting a string only during the
syscall handler scope, because doing an heap allocation is wasteful on memory resources
and puts a strain on the kernel memory manager for no good reason.
The FixedStringBuffer puts some safety guards - like zeroing the memory when storing new
StringView, as well as truncating it if its length exceeds the allocated stack storage size.
It should be noted that there are helpers to handle a FixedStringBuffer storage:
* `Process::get_syscall_name_string_argument_into_static_char_buffer(...)`
* `Process::get_syscall_string_argument_into_static_char_buffer(...)`
* `try_copy_name_from_user_into_static_char_buffer(...)`
* `try_copy_string_from_user_into_static_char_buffer(...)`
These helpers will ensure that if the given string is exceeding the allocated stack storage size, then an error will be released instead of just truncating the string and continue execution.
## We don't break userspace - the SerenityOS version ## We don't break userspace - the SerenityOS version
We don't break userspace. However, in contrast to the Linux vision on this statement, We don't break userspace. However, in contrast to the Linux vision on this statement,