lib/ukalloc: Fix underallocation bug in malloc

[andreittr]

Description of changes

This change fixes a logic error that allocates less memory than necessary in specific cases, risking the memory corruption of internal data structures, leading to unpredictable behavior and crashes.

The bug manifests when requesting exactly 24 bytes less than an integer number of pages (4072, 8168, etc.) because our malloc code reserves 24 bytes of extra space for bookkeeping metadata. However, the same malloc implementation returns buffers 32 bytes above an allocated page boundary, leading to 8 bytes being inadvertently underallocated. Any code that rightfully uses the entire buffer returned by malloc risks corrupting the following page, with dire consequences.
This fix reserves the correct amount of space when computing how many pages to allocate.

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Ran the checkpatch.uk on your commit series before opening this PR;
• Updated relevant documentation.

Base target

• Architecture(s): N/A
• Platform(s): N/A
• Application(s): N/A

Additional configuration

Minimal snippet to reproduce the issue:

{
    long *p = malloc(4072);
    p[508] = -1;
    free(p);
}

[John-Ted]

With this PR, the native port of redis now functions correctly. Previously, I would get crashes if I sent a large number of requests.

Reviewed-by: Ioan-Teodor Teugea ioan_teodor.teugea@stud.acs.upb.ro

[Starnox]

Nice catch! Tested it throughly!

Reviewed-by: Eduard-Florin Mihailescu mihailescu.eduard@gmail.com

[razvand]

Approved-by: Razvan Deaconescu razvand@unikraft.io