linux-kernel - gcc inserts __builtin_popcount, causes 'modpost: "_

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Date:   Mon, 11 Apr 2022 22:08:15 +0100
From:   Sergei Trofimovich <slyich@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     amd-gfx@...ts.freedesktop.org, Joe Perches <joe@...ches.com>,
        linux-kbuild@...r.kernel.org, Jakub Jelinek <jakub@...hat.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: gcc inserts __builtin_popcount, causes 'modpost: "__popcountdi2" ...
 amdgpu.ko] undefined'

Current linux-5.17.1 on fresh gcc-12 fails to build with errors like:

    ERROR: modpost: "__popcountdi2" [drivers/net/ethernet/broadcom/bnx2x/bnx2x.ko] undefined!
    ERROR: modpost: "__popcountdi2" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!

It is occasionally seen by others on previous gcc versions as well:

    https://lkml.org/lkml/2021/7/11/261
    https://lkml.org/lkml/2018/10/24/403

'__popcountdi2' are inserted by gcc for code like the following
from 'drivers/gpu/drm/amd/display/modules/hdcp/hdcp1_execution.c':

    static inline enum mod_hdcp_status validate_bksv(struct mod_hdcp *hdcp)
    {
        uint64_t n = 0;
        uint8_t count = 0;
        u8 bksv[sizeof(n)] = { };

        memcpy(bksv, hdcp->auth.msg.hdcp1.bksv, sizeof(hdcp->auth.msg.hdcp1.bksv));
        n = *(uint64_t *)bksv;

        /* Here gcc inserts 'count = __builtin_popcount(n);' */
        while (n) {
                count++;
                n &= (n - 1);
        }

        return (count == 20) ? MOD_HDCP_STATUS_SUCCESS :
                               MOD_HDCP_STATUS_HDCP1_INVALID_BKSV;
    }

Note that gcc can insert it regardless of -mno-* options.

How should linux.git handle it? A few options come to mind:

- Perhaps use libgcc.a directly.
- Just implement '__popcountdi2'. Example definition from libgcc:
  https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libgcc/libgcc2.c;hb=HEAD#l846
- Or workaround it with -fno-builtin-popcount in Makefiles.

CCing people who can help routing it and/or deciding on the fix:
amd-gfx@...ts.freedesktop.org, Joe Perches, linux-kbuild@...r.kernel.org,
Jakub Jelinek, Segher Boessenkool, Thomas Gleixner,Peter Zijlstra, Andy
Lutomirski.

WDYT?

Thanks!

-- 

  Sergei