Re: [PATCH] ARM: Add HAVE_FUNCTION_ARG_ACCESS_API support

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Mon, Jun 03, 2024 at 03:37:51PM +0800, Jinjie Ruan wrote:
> Currently, kprobe on ARM32 can not use the '$argx' syntax available on
> other architecture. So implement regs_get_kernel_argument() and add
> HAVE_FUNCTION_ARG_ACCESS_API support.

This may work in the simple case, but it just doesn't work in the
general case, where a function accepts 64-bit arguments. For example,
for EABI and a function taking a 64-bit argument followed by a 32-bit
argument:

	R0/R1 = 64-bit argument
	R2 = 32-bit argument

Now consider 32-bit argument followed by 64-bit argument:

	R0 = 32-bit argument
	R1 = unused
	R2/R3 = 64-bit argument

Note that the mapping isn't argN = RN.

Also, given that "unsigned long" is 32-bit on 32-bit Arm, one can't
return a 64-bit argument through this interface. Even if one typed
the function as u64, it still wouldn't work because the caller
assigns the return value to an unsigned long. This seems to be an
issue throughout the kernel tracing - it isn't written to support
64-bit arguments on 32-bit architectures. See, for example,
fetch_store_raw(), where the unsigned long gets cast to a u64.
It'll still only have 32-bits of significant value.

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