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Documentation: Update the kernel development guide on FixedStringBuffer

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The following changes take effect:
1. Annotate FixedStringBuffer => `FixedStringBuffer` in many places.
2. Remove non-existing helpers for FixedStringBuffers. I added them
previously but even then they were removed in a following fixup so these
references were never valid. Therefore let's just put a vague reference
to the fact that we have some helpers for this class in the Kernel, and
let people to figure out quickly by themselves for this topic.
3. Put a sentence to explain that FixedStringBuffer objects are not only
being used in syscall handling code, but also for storing actual data in
both the Thread and Process classes as well.
This commit is contained in:
Liav A 2023-08-11 20:34:02 +03:00 committed by Andrew Kaster
parent 2b23f3f216
commit b096f39e92
Notes: sideshowbarker 2024-07-17 06:45:52 +09:00 
Author: https://github.com/supercomputer7
Commit: https://github.com/SerenityOS/serenity/commit/b096f39e92
Pull-request: https://github.com/SerenityOS/serenity/pull/20500
1 changed files with 8 additions and 9 deletions
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17
Documentation/Kernel/DevelopmentGuidelines.md
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@ -42,7 +42,7 @@ As you might understand, we put a respectable amount of effort into making the k
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
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
@ -52,16 +52,15 @@ to create a KString. This is especially useful when inspecting a string only dur
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
The `Process` and `Thread` classes use a `FixedStringBuffer` to store their names,
to completely circumvent OOM conditions due to needing to allocate heap storage
for their names in the past.
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.
There are many helpers (in the `Process` class and also in the `Kernel/Library/StdLib.h` file) that will do a check on whether the input size is exceeding the allocated `FixedStringBuffer` storage size.
An appropriate error will be released instead of just truncating the string and continue execution in these helpers.
## We don't break userspace - the SerenityOS version