linux-kernel - [PATCH] recordmcount: only record relocation of type R_AARCH64

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Message-Id: <20200717143338.19302-1-gregory.herrero@oracle.com>
Date:   Fri, 17 Jul 2020 16:33:38 +0200
From:   gregory.herrero@...cle.com
To:     linux-kernel@...r.kernel.org
Cc:     rostedt@...dmis.org, stable@...r.kernel.org,
        Gregory Herrero <gregory.herrero@...cle.com>
Subject: [PATCH] recordmcount: only record relocation of type R_AARCH64_CALL26 on arm64.

From: Gregory Herrero <gregory.herrero@...cle.com>

Currently, if a section has a relocation to '_mcount' symbol, a new
__mcount_loc entry will be added whatever the relocation type is.
This is problematic when a relocation to '_mcount' is in the middle of a
section and is not a call for ftrace use.

Such relocation could be generated with below code for example:
    bool is_mcount(unsigned long addr)
    {
        return (target == (unsigned long) &_mcount);
    }

With this snippet of code, ftrace will try to patch the mcount location
generated by this code on module load and fail with:

    Call trace:
     ftrace_bug+0xa0/0x28c
     ftrace_process_locs+0x2f4/0x430
     ftrace_module_init+0x30/0x38
     load_module+0x14f0/0x1e78
     __do_sys_finit_module+0x100/0x11c
     __arm64_sys_finit_module+0x28/0x34
     el0_svc_common+0x88/0x194
     el0_svc_handler+0x38/0x8c
     el0_svc+0x8/0xc
    ---[ end trace d828d06b36ad9d59 ]---
    ftrace failed to modify
    [<ffffa2dbf3a3a41c>] 0xffffa2dbf3a3a41c
     actual:   66:a9:3c:90
    Initializing ftrace call sites
    ftrace record flags: 2000000
     (0)
    expected tramp: ffffa2dc6cf66724

So Limit the relocation type to R_AARCH64_CALL26 as in perl version of
recordmcount.

Fixes: ed60453fa8f8 ("ARM: 6511/1: ftrace: add ARM support for C version of recordmcount")
Signed-off-by: Gregory Herrero <gregory.herrero@...cle.com>
---
 scripts/recordmcount.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
index 7225107a9aaf..e59022b3f125 100644
--- a/scripts/recordmcount.c
+++ b/scripts/recordmcount.c
@@ -434,6 +434,11 @@ static int arm_is_fake_mcount(Elf32_Rel const *rp)
 	return 1;
 }
 
+static int arm64_is_fake_mcount(Elf64_Rel const *rp)
+{
+	return ELF64_R_TYPE(w(rp->r_info)) != R_AARCH64_CALL26;
+}
+
 /* 64-bit EM_MIPS has weird ELF64_Rela.r_info.
  * http://techpubs.sgi.com/library/manuals/4000/007-4658-001/pdf/007-4658-001.pdf
  * We interpret Table 29 Relocation Operation (Elf64_Rel, Elf64_Rela) [p.40]
@@ -547,6 +552,7 @@ static int do_file(char const *const fname)
 		make_nop = make_nop_arm64;
 		rel_type_nop = R_AARCH64_NONE;
 		ideal_nop = ideal_nop4_arm64;
+		is_fake_mcount64 = arm64_is_fake_mcount;
 		break;
 	case EM_IA_64:	reltype = R_IA64_IMM64; break;
 	case EM_MIPS:	/* reltype: e_class    */ break;
-- 
2.27.0