MIPS: Initial implementation of a VDSO

Add an initial implementation of a proper (i.e. an ELF shared library)
VDSO. With this commit it does not export any symbols, it only replaces
the current signal return trampoline page. A later commit will add user
implementations of gettimeofday()/clock_gettime().

To support both new toolchains and old ones which don't generate ABI
flags section, we define its content manually and then use a tool
(genvdso) to patch up the section to have the correct name and type.
genvdso also extracts symbol offsets ({,rt_}sigreturn) needed by the
kernel, and generates a C file containing a "struct mips_vdso_image"
containing both the VDSO data and these offsets. This C file is
compiled into the kernel.

On 64-bit kernels we require a different VDSO for each supported ABI,
so we may build up to 3 different VDSOs. The VDSO to use is selected by
the mips_abi structure.

A kernel/user shared data page is created and mapped below the VDSO
image. This is currently empty, but will be used by the user time
function implementations which are added later.

[markos.chandras@imgtec.com:
- Add more comments
- Move abi detection in genvdso.h since it's the get_symbol function
that needs it.
- Add an R6 specific way to calculate the base address of VDSO in order
to avoid the branch instruction which affects performance.
- Do not patch .gnu.attributes since it's not needed for dynamic linking.
- Simplify Makefile a little bit.
- checkpatch fixes
- Restrict VDSO support for binutils < 2.25 for pre-R6
- Include atomic64.h for O32 variant on MIPS64]

Signed-off-by: Alex Smith <alex.smith@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: Matthew Fortune <matthew.fortune@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11337/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 187 insertions, 0 deletions

diff --git a/arch/mips/vdso/genvdso.h b/arch/mips/vdso/genvdso.h
new file mode 100644
index 000000000000..94334727059a
--- /dev/null
+++ b/arch/mips/vdso/genvdso.h
@@ -0,0 +1,187 @@
+/*
+ * Copyright (C) 2015 Imagination Technologies
+ * Author: Alex Smith <alex.smith@imgtec.com>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation;  either version 2 of the  License, or (at your
+ * option) any later version.
+ */
+
+static inline bool FUNC(patch_vdso)(const char *path, void *vdso)
+{
+	const ELF(Ehdr) *ehdr = vdso;
+	void *shdrs;
+	ELF(Shdr) *shdr;
+	char *shstrtab, *name;
+	uint16_t sh_count, sh_entsize, i;
+	unsigned int local_gotno, symtabno, gotsym;
+	ELF(Dyn) *dyn = NULL;
+
+	shdrs = vdso + FUNC(swap_uint)(ehdr->e_shoff);
+	sh_count = swap_uint16(ehdr->e_shnum);
+	sh_entsize = swap_uint16(ehdr->e_shentsize);
+
+	shdr = shdrs + (sh_entsize * swap_uint16(ehdr->e_shstrndx));
+	shstrtab = vdso + FUNC(swap_uint)(shdr->sh_offset);
+
+	for (i = 0; i < sh_count; i++) {
+		shdr = shdrs + (i * sh_entsize);
+		name = shstrtab + swap_uint32(shdr->sh_name);
+
+		/*
+		 * Ensure there are no relocation sections - ld.so does not
+		 * relocate the VDSO so if there are relocations things will
+		 * break.
+		 */
+		switch (swap_uint32(shdr->sh_type)) {
+		case SHT_REL:
+		case SHT_RELA:
+			fprintf(stderr,
+				"%s: '%s' contains relocation sections\n",
+				program_name, path);
+			return false;
+		case SHT_DYNAMIC:
+			dyn = vdso + FUNC(swap_uint)(shdr->sh_offset);
+			break;
+		}
+
+		/* Check for existing sections. */
+		if (strcmp(name, ".MIPS.abiflags") == 0) {
+			fprintf(stderr,
+				"%s: '%s' already contains a '.MIPS.abiflags' section\n",
+				program_name, path);
+			return false;
+		}
+
+		if (strcmp(name, ".mips_abiflags") == 0) {
+			strcpy(name, ".MIPS.abiflags");
+			shdr->sh_type = swap_uint32(SHT_MIPS_ABIFLAGS);
+			shdr->sh_entsize = shdr->sh_size;
+		}
+	}
+
+	/*
+	 * Ensure the GOT has no entries other than the standard 2, for the same
+	 * reason we check that there's no relocation sections above.
+	 * The standard two entries are:
+	 * - Lazy resolver
+	 * - Module pointer
+	 */
+	if (dyn) {
+		local_gotno = symtabno = gotsym = 0;
+
+		while (FUNC(swap_uint)(dyn->d_tag) != DT_NULL) {
+			switch (FUNC(swap_uint)(dyn->d_tag)) {
+			/*
+			 * This member holds the number of local GOT entries.
+			 */
+			case DT_MIPS_LOCAL_GOTNO:
+				local_gotno = FUNC(swap_uint)(dyn->d_un.d_val);
+				break;
+			/*
+			 * This member holds the number of entries in the
+			 * .dynsym section.
+			 */
+			case DT_MIPS_SYMTABNO:
+				symtabno = FUNC(swap_uint)(dyn->d_un.d_val);
+				break;
+			/*
+			 * This member holds the index of the first dynamic
+			 * symbol table entry that corresponds to an entry in
+			 * the GOT.
+			 */
+			case DT_MIPS_GOTSYM:
+				gotsym = FUNC(swap_uint)(dyn->d_un.d_val);
+				break;
+			}
+
+			dyn++;
+		}
+
+		if (local_gotno > 2 || symtabno - gotsym) {
+			fprintf(stderr,
+				"%s: '%s' contains unexpected GOT entries\n",
+				program_name, path);
+			return false;
+		}
+	}
+
+	return true;
+}
+
+static inline bool FUNC(get_symbols)(const char *path, void *vdso)
+{
+	const ELF(Ehdr) *ehdr = vdso;
+	void *shdrs, *symtab;
+	ELF(Shdr) *shdr;
+	const ELF(Sym) *sym;
+	char *strtab, *name;
+	uint16_t sh_count, sh_entsize, st_count, st_entsize, i, j;
+	uint64_t offset;
+	uint32_t flags;
+
+	shdrs = vdso + FUNC(swap_uint)(ehdr->e_shoff);
+	sh_count = swap_uint16(ehdr->e_shnum);
+	sh_entsize = swap_uint16(ehdr->e_shentsize);
+
+	for (i = 0; i < sh_count; i++) {
+		shdr = shdrs + (i * sh_entsize);
+
+		if (swap_uint32(shdr->sh_type) == SHT_SYMTAB)
+			break;
+	}
+
+	if (i == sh_count) {
+		fprintf(stderr, "%s: '%s' has no symbol table\n", program_name,
+			path);
+		return false;
+	}
+
+	/* Get flags */
+	flags = swap_uint32(ehdr->e_flags);
+	if (elf_class == ELFCLASS64)
+		elf_abi = ABI_N64;
+	else if (flags & EF_MIPS_ABI2)
+		elf_abi = ABI_N32;
+	else
+		elf_abi = ABI_O32;
+
+	/* Get symbol table. */
+	symtab = vdso + FUNC(swap_uint)(shdr->sh_offset);
+	st_entsize = FUNC(swap_uint)(shdr->sh_entsize);
+	st_count = FUNC(swap_uint)(shdr->sh_size) / st_entsize;
+
+	/* Get string table. */
+	shdr = shdrs + (swap_uint32(shdr->sh_link) * sh_entsize);
+	strtab = vdso + FUNC(swap_uint)(shdr->sh_offset);
+
+	/* Write offsets for symbols needed by the kernel. */
+	for (i = 0; vdso_symbols[i].name; i++) {
+		if (!(vdso_symbols[i].abis & elf_abi))
+			continue;
+
+		for (j = 0; j < st_count; j++) {
+			sym = symtab + (j * st_entsize);
+			name = strtab + swap_uint32(sym->st_name);
+
+			if (!strcmp(name, vdso_symbols[i].name)) {
+				offset = FUNC(swap_uint)(sym->st_value);
+
+				fprintf(out_file,
+					"\t.%s = 0x%" PRIx64 ",\n",
+					vdso_symbols[i].offset_name, offset);
+				break;
+			}
+		}
+
+		if (j == st_count) {
+			fprintf(stderr,
+				"%s: '%s' is missing required symbol '%s'\n",
+				program_name, path, vdso_symbols[i].name);
+			return false;
+		}
+	}
+
+	return true;
+}