[PATCH] elf: add AT_ARGV and AT_ENVV auxiliary vector entries

[ahajkova]

From: Alexandra Hájková <ahajkova@xxxxxxxxxx>

Add AT_ARGV (52), which contains the address of the argv pointer
array on the initial process stack, and AT_ENVV (53), which contains
the address of the envp pointer array.

Add a kselftest (tools/testing/selftests/exec/test_auxv) that reads
/proc/self/auxv and verifies AT_ARGV and AT_ENVV point to the process's
own argv[] and envp[].

The motivation is to allow GDB to find argv and envp in core dumps
cleanly. Currently GDB locates them by scanning backwards through
stack memory from AT_EXECFN, which is fragile. With AT_ARGV and
AT_ENVV it can read the addresses directly from the core dump's
auxv note.

Signed-off-by: Alexandra Hájková <ahajkova@xxxxxxxxxx>
---
 fs/binfmt_elf.c                          |  9 +++++-
 include/linux/auxvec.h                   |  2 +-
 include/uapi/linux/auxvec.h              |  2 ++
 tools/testing/selftests/exec/Makefile    |  1 +
 tools/testing/selftests/exec/test_auxv.c | 37 ++++++++++++++++++++++++
 5 files changed, 49 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/exec/test_auxv.c

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 8e89cc5b2820..6a0830a63335 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -182,6 +182,8 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
 	int ei_index;
 	const struct cred *cred = current_cred();
 	struct vm_area_struct *vma;
+	elf_addr_t *at_argv_val;
+	elf_addr_t *at_envv_val;
 
 	/*
 	 * In some cases (e.g. Hyper-Threading), we want to avoid L1
@@ -288,6 +290,10 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
 	NEW_AUX_ENT(AT_RSEQ_FEATURE_SIZE, offsetof(struct rseq, end));
 	NEW_AUX_ENT(AT_RSEQ_ALIGN, __alignof__(struct rseq));
 #endif
+	at_argv_val = elf_info + 1;
+	NEW_AUX_ENT(AT_ARGV, 0);
+	at_envv_val = elf_info + 1;
+	NEW_AUX_ENT(AT_ENVV, 0);
 #undef NEW_AUX_ENT
 	/* AT_NULL is zero; clear the rest too */
 	memset(elf_info, 0, (char *)mm->saved_auxv +
@@ -309,7 +315,8 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
 #else
 	sp = (elf_addr_t __user *)bprm->p;
 #endif
-
+	*at_argv_val = (unsigned long)(sp + 1);
+	*at_envv_val = (unsigned long)(sp + (argc + 2));
 
 	/*
 	 * Grow the stack manually; some architectures have a limit on how
diff --git a/include/linux/auxvec.h b/include/linux/auxvec.h
index 8bcb9b726262..7184de95b89d 100644
--- a/include/linux/auxvec.h
+++ b/include/linux/auxvec.h
@@ -4,6 +4,6 @@
 
 #include <uapi/linux/auxvec.h>
 
-#define AT_VECTOR_SIZE_BASE 24 /* NEW_AUX_ENT entries in auxiliary table */
+#define AT_VECTOR_SIZE_BASE 26 /* NEW_AUX_ENT entries in auxiliary table */
   /* number of "#define AT_.*" above, minus {AT_NULL, AT_IGNORE, AT_NOTELF} */
 #endif /* _LINUX_AUXVEC_H */
diff --git a/include/uapi/linux/auxvec.h b/include/uapi/linux/auxvec.h
index cc61cb9b3e9a..e7af32709aba 100644
--- a/include/uapi/linux/auxvec.h
+++ b/include/uapi/linux/auxvec.h
@@ -40,5 +40,7 @@
 #ifndef AT_MINSIGSTKSZ
 #define AT_MINSIGSTKSZ	51	/* minimal stack size for signal delivery */
 #endif
+#define AT_ARGV		52	/* address of argv[] */
+#define AT_ENVV		53	/* address of envp[] */
 
 #endif /* _UAPI_LINUX_AUXVEC_H */
diff --git a/tools/testing/selftests/exec/Makefile b/tools/testing/selftests/exec/Makefile
index 45a3cfc435cf..4370a0a65d22 100644
--- a/tools/testing/selftests/exec/Makefile
+++ b/tools/testing/selftests/exec/Makefile
@@ -20,6 +20,7 @@ TEST_FILES := Makefile
 TEST_GEN_PROGS += recursion-depth
 TEST_GEN_PROGS += null-argv
 TEST_GEN_PROGS += check-exec
+TEST_GEN_PROGS += test_auxv
 
 EXTRA_CLEAN := $(OUTPUT)/subdir.moved $(OUTPUT)/execveat.moved $(OUTPUT)/xxxxx*	\
 	       $(OUTPUT)/S_I*.test
diff --git a/tools/testing/selftests/exec/test_auxv.c b/tools/testing/selftests/exec/test_auxv.c
new file mode 100644
index 000000000000..5d1215562d40
--- /dev/null
+++ b/tools/testing/selftests/exec/test_auxv.c
@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <stdio.h>
+#include <stdlib.h>
+#include <linux/auxvec.h>
+#include "kselftest.h"
+
+int main(int argc, char **argv, char **envp)
+{
+	FILE *file = fopen("/proc/self/auxv", "rb");
+	int argv_found = 0;
+	int envp_found = 0;
+	struct {
+		unsigned long type;
+		unsigned long val;
+	} entry;
+
+	if (file == NULL)
+		return 1;
+
+	ksft_print_header();
+	ksft_set_plan(1);
+	while (fread(&entry, sizeof(entry), 1, file) == 1) {
+		if (entry.type == 0)
+			break;
+		if (entry.type == AT_ARGV && entry.val == (unsigned long)argv)
+			argv_found++;
+		if (entry.type == AT_ENVV && entry.val == (unsigned long)envp)
+			envp_found++;
+	}
+	if (argv_found && envp_found)
+		ksft_test_result_pass("AT_ARGV and AT_ENVV\n");
+	else
+		ksft_test_result_fail("AT_ARGV and AT_ENVV\n");
+
+	fclose(file);
+	ksft_finished();
+}
-- 
2.47.3