Typecasting a void pointer to unsigned long in zsmalloc.c

[Leam Hall]

On 1/24/25 15:45, Sotir Danailov wrote:
> I found this page which looks a bit legacy and doesn't seem to be
> linked on kernel.org anywhere: https://www.kernel.org/doc/
> In here, under "Standards documents applicable to the Linux kernel",
> there's an interesting line:
> "LP64 standard defining the size of char, short, int, and long on
> 32-bit and 64-bit platforms." and it links to this:
> https://unix.org/whitepapers/64bit.html
> In the link, under "64-bit Data Models", there's a table, which shows
> that LP64 means:
> char 8 bits, short 16 bits, int 32 bits, long 64 bits, pointer 64 bits
> 
> This lead me to:
> https://gitlab.com/x86-psABIs/x86-64-ABI/-/jobs/artifacts/master/raw/x86-64-ABI/abi.pdf?job=build
> which explains that for "System V AMD64 ABI":
> on page 12: "all pointer types are 64-bit objects (LP64)"
> on page 17: there's a table showing that "unsigned long (LP64)" is 8 bytes.
> 
> Finally, in GCC's manual for x86:
> https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html#index-mabi-7
> "The default is to use the Microsoft ABI when targeting Microsoft
> Windows and the SysV ABI on all other systems."
> 
> I was wondering how it worked for other architectures, so I decided to
> grep the Makefiles in the Linux tree and apparently for some
> architectures it's explicitly stated:
> ./arch/arm64/Makefile:53:KBUILD_CFLAGS    += $(call cc-option,-mabi=lp64)
> ./arch/riscv/Makefile:33:    KBUILD_CFLAGS += -mabi=lp64
> 
> What I found in kernelnewbies: https://kernelnewbies.org/InternalKernelDataTypes
> "One thing that has caused a lot of problems, as 64-bit machines are
> getting more popular, is the fact that the size of a pointer is not
> the same as the size of an unsigned integer. The size of a pointer is
> equal to the size of an unsigned long."
> 
> I think there should be an explanation both on kernel.org and
> kernelnewbies what determines the sizes, so that people can have a
> better idea of what's going on. It should be something that is easy to
> find and part of "introductions" to kernel development, because it's
> not obvious at all how all of these things are setup. To me it seems
> fundamental, to be able to quickly find easy to digest information
> regarding the data types you're working with.


Sotir, great research!

Sadly, https://kernelnewbies.org is giving a "ConfigurationError".  :(  So I'm not sure if the reference to https://www.kernel.org/doc/ is already in place, or not. Something to check when KN is back up.

I'd think pointing to good documents is better than replicating them and risk staleness, isn't it?

Leam


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