[PATCH v3] elf loader support for auxvec base platform string

Nathan Lynch
2008-07-17T22:19:50Z

Some IBM POWER-based platforms have the ability to run in a
mode which mostly appears to the OS as a different processor from the
actual hardware. For example, a Power6 system may appear to be a
Power5+, which makes the AT_PLATFORM value "power5+". This means that
programs are restricted to the ISA supported by Power5+;
Power6-specific instructions are treated as illegal.

However, some applications (virtual machines, optimized libraries) can
benefit from knowledge of the underlying CPU model. A new aux vector
entry, AT_BASE_PLATFORM, will denote the actual hardware. For
example, on a Power6 system in Power5+ compatibility mode, AT_PLATFORM
will be "power5+" and AT_BASE_PLATFORM will be "power6". The idea is
that AT_PLATFORM indicates the instruction set supported, while
AT_BASE_PLATFORM indicates the underlying microarchitecture.

If the architecture has defined ELF_BASE_PLATFORM, copy that value to
the user stack in the same manner as ELF_PLATFORM.

Signed-off-by: Nathan Lynch <ntl@pobox.com>

---

Added comment explaining ELF_BASE_PLATFORM.

fs/binfmt_elf.c | 28 ++++++++++++++++++++++++++++
include/linux/auxvec.h | 5 ++++-
2 files changed, 32 insertions(+), 1 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index d48ff5f..d8a7cc0 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -131,6 +131,15 @@ static int padzero(unsigned long elf_bss)
#define STACK_ALLOC(sp, len) ({ sp -= len ; sp; })
#endif

+#ifndef ELF_BASE_PLATFORM
+/*
+ * AT_BASE_PLATFORM indicates the "real" hardware/microarchitecture.
+ * If the arch defines ELF_BASE_PLATFORM (in asm/elf.h), the value
+ * will be copied to the user stack in the same manner as AT_PLATFORM.
+ */
+#define ELF_BASE_PLATFORM NULL
+#endif
+
static int
create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
unsigned long load_addr, unsigned long interp_load_addr)
@@ -142,7 +151,9 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
elf_addr_t __user *envp;
elf_addr_t __user *sp;
elf_addr_t __user *u_platform;
+ elf_addr_t __user *u_base_platform;
const char *k_platform = ELF_PLATFORM;
+ const char *k_base_platform = ELF_BASE_PLATFORM;
int items;
elf_addr_t *elf_info;
int ei_index = 0;
@@ -172,6 +183,19 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
return -EFAULT;
}

+ /*
+ * If this architecture has a "base" platform capability
+ * string, copy it to userspace.
+ */
+ u_base_platform = NULL;
+ if (k_base_platform) {
+ size_t len = strlen(k_base_platform) + 1;
+
+ u_base_platform = (elf_addr_t __user *)STACK_ALLOC(p, len);
+ if (__copy_to_user(u_base_platform, k_base_platform, len))
+ return -EFAULT;
+ }
+
/* Create the ELF interpreter info */
elf_info = (elf_addr_t *)current->mm->saved_auxv;
/* update AT_VECTOR_SIZE_BASE if the number of NEW_AUX_ENT() changes */
@@ -208,6 +232,10 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
NEW_AUX_ENT(AT_PLATFORM,
(elf_addr_t)(unsigned long)u_platform);
}
+ if (k_base_platform) {
+ NEW_AUX_ENT(AT_BASE_PLATFORM,
+ (elf_addr_t)(unsigned long)u_base_platform);
+ }
if (bprm->interp_flags & BINPRM_FLAGS_EXECFD) {
NEW_AUX_ENT(AT_EXECFD, bprm->interp_data);
}
diff --git a/include/linux/auxvec.h b/include/linux/auxvec.h
index ad89545..1adc61d 100644
--- a/include/linux/auxvec.h
+++ b/include/linux/auxvec.h
@@ -26,8 +26,11 @@

#define AT_SECURE 23 /* secure mode boolean */

+#define AT_BASE_PLATFORM 38 /* string identifying real platform, may
+ * differ from AT_PLATFORM. */
+
#ifdef __KERNEL__
-#define AT_VECTOR_SIZE_BASE (14 + 2) /* NEW_AUX_ENT entries in auxiliary table */
+#define AT_VECTOR_SIZE_BASE (14 + 3) /* NEW_AUX_ENT entries in auxiliary table */
#endif

#endif /* _LINUX_AUXVEC_H */
--
1.5.6.2