linux-kernel - [PATCH 2/3] modpost: use more reliable way to get fromsec in section

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Date:   Sun, 31 Jul 2022 02:36:35 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     linux-kbuild@...r.kernel.org
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/3] modpost: use more reliable way to get fromsec in section_rel(a)()

The section name of Rel and Rela starts with ".rel" and ".rela"
respectively (but, I do not know whether this is specification or
convention).

For example, ".rela.text" holds relocation entries applied to the
".text" section.

So, the code chops the ".rel" or ".rela" prefix to get the name of
the section to which the relocation applies.

However, I do not like to skip 4 or 5 bytes blindly because it is
potential memory overrun.

The ELF specification provides a more reliable way to do this.

 - The sh_info field holds extra information, whose interpretation
   depends on the section type

 - If the section type is SHT_REL or SHT_RELA, the sh_info field holds
   the section header index of the section to which the relocation
   applies.

Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
---

 scripts/mod/modpost.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 148b38699889..c6a055c0291e 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -1723,8 +1723,7 @@ static void section_rela(const char *modname, struct elf_info *elf,
 	Elf_Rela *start = (void *)elf->hdr + sechdr->sh_offset;
 	Elf_Rela *stop  = (void *)start + sechdr->sh_size;
 
-	fromsec = sech_name(elf, sechdr);
-	fromsec += strlen(".rela");
+	fromsec = sec_name(elf, sechdr->sh_info);
 	/* if from section (name) is know good then skip it */
 	if (match(fromsec, section_white_list))
 		return;
@@ -1776,8 +1775,7 @@ static void section_rel(const char *modname, struct elf_info *elf,
 	Elf_Rel *start = (void *)elf->hdr + sechdr->sh_offset;
 	Elf_Rel *stop  = (void *)start + sechdr->sh_size;
 
-	fromsec = sech_name(elf, sechdr);
-	fromsec += strlen(".rel");
+	fromsec = sec_name(elf, sechdr->sh_info);
 	/* if from section (name) is know good then skip it */
 	if (match(fromsec, section_white_list))
 		return;
-- 
2.34.1