Re: [PATCH v6 2/3] rust: io: mem: add a generic iomem abstraction

[Joel Fernandes <joelagnelf@nvidia.com>]

On Thu, Feb 06, 2025 at 12:57:30PM -0300, Daniel Almeida wrote:
> Btw, Miguel & others,
> 
> IMHO, I think we should write a comment about this somewhere in the docs.
> 
> When I first came across this issue myself, it took me a while to understand that
> the build_error was actually triggering.
> 
> That’s because the result is:
> 
> ```
> ERROR: modpost: "rust_build_error" [rust_platform_uio_driver.ko] undefined!
> ```
> 
> When a symbol is undefined, someone would be within their rights to assume that
> something is broken in some KConfig somewhere, like this person did. It specifically
> doesn’t tell them that the problem is their own code triggering a build_error because
> they are misusing an API.
> 
> I know that we can’t really provide a message through build_error itself, hence my
> suggestion about the docs.
> 
> I can send a patch if you agree, it will prevent this confusion from coming up in the
> future.

Interesting, I just ran into this. I am writing function as follows that
reads bar0 in the nova driver, however not having the "if current_len + i"
causes the same issue:

ERROR: modpost: "rust_build_error" [nova_core.ko] undefined!

which did not help much about what the issue really is. I had to figure it
out through tedious trial and error. Also what is the reason for this, the
compiler is doing some checks in with_bar? Looking at with_bar
implementation, I could not see any. Also enabling
CONFIG_RUST_BUILD_ASSERT_ALLOW did not show more menaingful messages. Thanks
for taking a look:

    pub(crate) fn read_more(&mut self, bytes: u32) -> Result {
        with_bar!(self.bar0, |bar0| {
            // Get current length
            let current_len = self.data.len();

            // Read ROM data bytes push directly to vector
            for i in 0..bytes as usize {

		// This check fixes:
		// ERROR: modpost: "rust_build_error" [nova_core.ko] undefined!
                if current_len + i >= 10000000 {
                    return Err(EINVAL);
                }

                // Read a byte from the VBIOS ROM and push it to the data vector
                let rom_addr = ROM_OFFSET + current_len + i;
                let byte = bar0.readb(rom_addr);
                self.data.push(byte, GFP_KERNEL)?;
            }

            Ok(())
        })?
    }

thanks,

 - Joel