linux-kernel - [PATCH 1/2] binfmt_flat: allow not offsetting data start

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Date:   Wed,  7 Apr 2021 20:56:37 +0900
From:   Damien Le Moal <damien.lemoal@....com>
To:     Palmer Dabbelt <palmer@...belt.com>,
        linux-riscv@...ts.infradead.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org
Cc:     Max Filippov <jcmvbkbc@...il.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Anup Patel <anup.patel@....com>, Christoph Hellwig <hch@....de>
Subject: [PATCH 1/2] binfmt_flat: allow not offsetting data start

Commit 2217b9826246 ("binfmt_flat: revert "binfmt_flat: don't offset
the data start"") restored offsetting the start of the data section by
a number of words defined by MAX_SHARED_LIBS. As a result, since
MAX_SHARED_LIBS is never 0, a gap between the text and data sections
always exist. For architecture which cannot support a such gap between
the text and data sections (e.g. riscv nommu), flat binary programs
cannot be executed.

To allow an architecture to request contiguous text and data sections,
introduce the macro FLAT_TEXT_DATA_NO_GAP which can be defined by the
architecture in its asm/flat.h file. With this change, the macro
DATA_GAP_WORDS is conditionally defined in binfmt_flat.c to
MAX_SHARED_LIBS for architectures tolerating the gap
(FLAT_TEXT_DATA_NO_GAP undefined case) and to 0 when
FLAT_TEXT_DATA_NO_GAP is defined. DATA_GAP_WORDS is used in
load_flat_file() to calculate the data section length and start
position.

The definition of FLAT_TEXT_DATA_NO_GAP by an architecture also
prevents the use of the separate text/data load case (when
FLAT_FLAG_RAM and FLAT_FLAG_GZIP are not set with NOMMU kernels).

Signed-off-by: Damien Le Moal <damien.lemoal@....com>
---
 fs/binfmt_flat.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c
index b9c658e0548e..2bfa05ac5cb4 100644
--- a/fs/binfmt_flat.c
+++ b/fs/binfmt_flat.c
@@ -74,6 +74,12 @@
 #define	MAX_SHARED_LIBS			(1)
 #endif
 
+#ifdef FLAT_TEXT_DATA_NO_GAP
+#define DATA_GAP_WORDS			(0)
+#else
+#define DATA_GAP_WORDS			(MAX_SHARED_LIBS)
+#endif
+
 struct lib_info {
 	struct {
 		unsigned long start_code;		/* Start of text segment */
@@ -437,7 +443,6 @@ static int load_flat_file(struct linux_binprm *bprm,
 	__be32 __user *reloc;
 	u32 __user *rp;
 	int i, rev, relocs;
-	loff_t fpos;
 	unsigned long start_code, end_code;
 	ssize_t result;
 	int ret;
@@ -560,6 +565,9 @@ static int load_flat_file(struct linux_binprm *bprm,
 	 * it all together.
 	 */
 	if (!IS_ENABLED(CONFIG_MMU) && !(flags & (FLAT_FLAG_RAM|FLAT_FLAG_GZIP))) {
+#ifndef FLAT_TEXT_DATA_NO_GAP
+		loff_t fpos;
+
 		/*
 		 * this should give us a ROM ptr,  but if it doesn't we don't
 		 * really care
@@ -576,7 +584,7 @@ static int load_flat_file(struct linux_binprm *bprm,
 			goto err;
 		}
 
-		len = data_len + extra + MAX_SHARED_LIBS * sizeof(unsigned long);
+		len = data_len + extra + DATA_GAP_WORDS * sizeof(unsigned long);
 		len = PAGE_ALIGN(len);
 		realdatastart = vm_mmap(NULL, 0, len,
 			PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE, 0);
@@ -591,7 +599,7 @@ static int load_flat_file(struct linux_binprm *bprm,
 			goto err;
 		}
 		datapos = ALIGN(realdatastart +
-				MAX_SHARED_LIBS * sizeof(unsigned long),
+				DATA_GAP_WORDS * sizeof(unsigned long),
 				FLAT_DATA_ALIGN);
 
 		pr_debug("Allocated data+bss+stack (%u bytes): %lx\n",
@@ -620,9 +628,14 @@ static int load_flat_file(struct linux_binprm *bprm,
 			(datapos + (ntohl(hdr->reloc_start) - text_len));
 		memp = realdatastart;
 		memp_size = len;
+#else
+		pr_err("Separate text/data loading not supported\n");
+		ret = -ENOEXEC;
+		goto err;
+#endif /* FLAT_TEXT_DATA_NO_GAP */
 	} else {
 
-		len = text_len + data_len + extra + MAX_SHARED_LIBS * sizeof(u32);
+		len = text_len + data_len + extra + DATA_GAP_WORDS * sizeof(u32);
 		len = PAGE_ALIGN(len);
 		textpos = vm_mmap(NULL, 0, len,
 			PROT_READ | PROT_EXEC | PROT_WRITE, MAP_PRIVATE, 0);
@@ -638,7 +651,7 @@ static int load_flat_file(struct linux_binprm *bprm,
 
 		realdatastart = textpos + ntohl(hdr->data_start);
 		datapos = ALIGN(realdatastart +
-				MAX_SHARED_LIBS * sizeof(u32),
+				DATA_GAP_WORDS * sizeof(u32),
 				FLAT_DATA_ALIGN);
 
 		reloc = (__be32 __user *)
@@ -714,7 +727,7 @@ static int load_flat_file(struct linux_binprm *bprm,
 			ret = result;
 			pr_err("Unable to read code+data+bss, errno %d\n", ret);
 			vm_munmap(textpos, text_len + data_len + extra +
-				MAX_SHARED_LIBS * sizeof(u32));
+				  DATA_GAP_WORDS * sizeof(u32));
 			goto err;
 		}
 	}
-- 
2.30.2