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Kernel: Use FixedStringBuffer for fixed-length strings in syscalls

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Using the kernel stack is preferable, especially when the examined
strings should be limited to a reasonable length.

This is a small improvement, because if we don't actually move these
strings then we don't need to own heap allocations for them during the
syscall handler function scope.

In addition to that, some kernel strings are known to be limited, like
the hostname string, for these strings we also can use FixedStringBuffer
to store and copy to and from these buffers, without using any heap
allocations at all.
This commit is contained in:
Liav A 2023-07-17 19:22:01 +03:00 • committed by Andrew Kaster
parent 3fd4997fc2
commit d8b514873f
13 changed files with 100 additions and 46 deletions
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16
Kernel/Syscalls/hostname.cpp
View file

@ -15,9 +15,15 @@ ErrorOr<FlatPtr> Process::sys$gethostname(Userspace<char*> buffer, size_t size)
if (size > NumericLimits<ssize_t>::max())
return EINVAL;
return hostname().with_shared([&](auto const& name) -> ErrorOr<FlatPtr> {
if (size < (name->length() + 1))
// NOTE: To be able to copy a null-terminated string, we need at most
// 65 characters to store and copy and not 64 here, to store the whole
// hostname string + null terminator.
FixedStringBuffer<UTSNAME_ENTRY_LEN> current_hostname {};
current_hostname.store_characters(name.representable_view());
auto name_view = current_hostname.representable_view();
if (size < (name_view.length() + 1))
return ENAMETOOLONG;
TRY(copy_to_user(buffer, name->characters(), name->length() + 1));
TRY(copy_to_user(buffer, name_view.characters_without_null_termination(), name_view.length() + 1));
return 0;
});
}
@ -30,11 +36,9 @@ ErrorOr<FlatPtr> Process::sys$sethostname(Userspace<char const*> buffer, size_t
auto credentials = this->credentials();
if (!credentials->is_superuser())
return EPERM;
if (length > 64)
return ENAMETOOLONG;
auto new_name = TRY(try_copy_kstring_from_user(buffer, length));
auto new_hostname = TRY(get_syscall_name_string_fixed_buffer<UTSNAME_ENTRY_LEN - 1>(buffer, length));
hostname().with_exclusive([&](auto& name) {
name = move(new_name);
name.store_characters(new_hostname.representable_view());
});
return 0;
}