[LTP] shmctl03.c is broken for 32bit compat mode

[Li Wang]

Hi Dan,

On Fri, Aug 8, 2025 at 9:53 PM Dan Carpenter <dan.carpenter@linaro.org>
wrote:

> In 32bit compat mode the shmctl03.c test will always fail:
>
> shmctl03.c:33: TFAIL: /proc/sys/kernel/shmmax != 2147483647 got 4294967295
> shmctl03.c:34: TPASS: /proc/sys/kernel/shmmni = 4096
> shmctl03.c:35: TFAIL: /proc/sys/kernel/shmall != 4278190079 got 4294967295
>
> The test basically does this:
> // === === ===
> #define _GNU_SOURCE
> #include <sys/shm.h>
> #include <stdio.h>
>
> int main(void)
> {
>         struct shminfo info;
>
>         shmctl(0, IPC_INFO, (struct shmid_ds *)&info);
>
>         printf("shmmax = %lu\n", info.shmmax);
>         printf("shmmni = %lu\n", info.shmmni);
>         printf("shmall = %lu\n", info.shmall);
>
>         return 0;
> }
> // === === ===
>
> It compares that output with what we read from the file.  You can run
> "gcc -m32 test.c && ./a.out" to see the issue.
>
> In the first line shmmax is not the value that we read from the file
> because it was capped at INT_MAX by the kernel in commit af7c693f1460
> ("Cap shmmax at INT_MAX in compat shminfo").
> https://elixir.bootlin.com/linux/v6.16/source/ipc/shm.c#L1347
>
> With the last line we're trying to store a u64 value into a u32.  We're
> going to lose something so it's not going to be accurate.  The difference
> is how scanf() truncates it.  If you have 32bit longs then it will give
> you the first u32 but if you assign a u64 to a u32 like the rest of the
> code does then you'll get the last 32 bits.
>
> What's the right way to go about fixing this?
>

Maybe we can simply split the comparison part into two:

  #ifdef TST_ABI64: go with the original way.

  #ifdef TST_ABI32:
      'shmmax' expects INT_MAX from shmctl(0, IPC_INFO, ...), even if '
/proc/sys/kernel/shmmax' is higher.
      'shmall' compares the lower 32 bits of the value (expect_shmall =
shmall & 0xFFFFFFFF;)


-- 
Regards,
Li Wang