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Peter Anvin" Subject: [PATCH v3 2/3] x86/alternative: Use a helper function for patching alternatives Date: Mon, 10 Nov 2025 09:23:38 +0100 Message-ID: <20251110082339.30144-3-jgross@suse.com> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20251110082339.30144-1-jgross@suse.com> References: <20251110082339.30144-1-jgross@suse.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Pre-Result: action=no action; module=replies; Message is reply to one we originated X-Rspamd-Queue-Id: 01FA033724 X-Rspamd-Pre-Result: action=no action; module=replies; Message is reply to one we originated X-Rspamd-Action: no action X-Spam-Flag: NO X-Spam-Score: -4.00 X-Spam-Level: X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Spamd-Result: default: False [-4.00 / 50.00]; REPLY(-4.00)[] Tidy up apply_alternatives() by moving the main patching action of a single alternative instance into a helper function. As the original instruction is being patched with optimized padding NOPs even in case no replacement instruction has been selected, do the main patching action in the main loop. This requires to use __apply_relocation() instead of text_poke_apply_relocation() in order to have only a single call of optimize_nops(), resulting in a more transparent coding (the question why optimize_nops() isn't being called in all cases has come up more than once in the past). Suggested-by: Borislav Petkov Signed-off-by: Juergen Gross --- V3: - new patch --- arch/x86/kernel/alternative.c | 121 +++++++++++++++++++--------------- 1 file changed, 67 insertions(+), 54 deletions(-) diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 4f3ea50e41e8..248e5f212a2b 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -604,6 +604,68 @@ static inline u8 * instr_va(struct alt_instr *i) return (u8 *)&i->instr_offset + i->instr_offset; } +struct patch_site { + u8 *instr; + u8 buff[MAX_PATCH_LEN]; + u8 len; +}; + +static void __init_or_module analyze_patch_site(struct patch_site *ps, + struct alt_instr *p, struct alt_instr *end) +{ + struct alt_instr *r; + u8 buff_sz; + u8 *repl; + + /* + * In case of nested ALTERNATIVE()s the outer alternative might add + * more padding. To ensure consistent patching find the max padding for + * all alt_instr entries for this site (nested alternatives result in + * consecutive entries). + */ + ps->instr = instr_va(p); + ps->len = p->instrlen; + for (r = p+1; r < end && instr_va(r) == ps->instr; r++) { + ps->len = max(ps->len, r->instrlen); + p->instrlen = r->instrlen = ps->len; + } + + BUG_ON(ps->len > sizeof(ps->buff)); + BUG_ON(p->cpuid >= (NCAPINTS + NBUGINTS) * 32); + + /* + * Patch if either: + * - feature is present + * - feature not present but ALT_FLAG_NOT is set to mean, + * patch if feature is *NOT* present. + */ + if (!boot_cpu_has(p->cpuid) == !(p->flags & ALT_FLAG_NOT)) { + memcpy(ps->buff, ps->instr, ps->len); + return; + } + + repl = (u8 *)&p->repl_offset + p->repl_offset; + DPRINTK(ALT, "feat: %d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d) flags: 0x%x", + p->cpuid >> 5, p->cpuid & 0x1f, + ps->instr, ps->instr, ps->len, + repl, p->replacementlen, p->flags); + + memcpy(ps->buff, repl, p->replacementlen); + buff_sz = p->replacementlen; + + if (p->flags & ALT_FLAG_DIRECT_CALL) + buff_sz = alt_replace_call(ps->instr, ps->buff, p); + + for (; buff_sz < ps->len; buff_sz++) + ps->buff[buff_sz] = 0x90; + + __apply_relocation(ps->buff, ps->instr, ps->len, repl, p->replacementlen); + + DUMP_BYTES(ALT, ps->instr, ps->len, "%px: old_insn: ", ps->instr); + DUMP_BYTES(ALT, repl, p->replacementlen, "%px: rpl_insn: ", repl); + DUMP_BYTES(ALT, ps->buff, ps->len, "%px: final_insn: ", ps->instr); +} + /* * Replace instructions with better alternatives for this CPU type. This runs * before SMP is initialized to avoid SMP problems with self modifying code. @@ -617,9 +679,7 @@ static inline u8 * instr_va(struct alt_instr *i) void __init_or_module noinline apply_alternatives(struct alt_instr *start, struct alt_instr *end) { - u8 insn_buff[MAX_PATCH_LEN]; - u8 *instr, *replacement; - struct alt_instr *a, *b; + struct alt_instr *a; DPRINTK(ALT, "alt table %px, -> %px", start, end); @@ -643,59 +703,12 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start, * order. */ for (a = start; a < end; a++) { - unsigned int insn_buff_sz = 0; - - /* - * In case of nested ALTERNATIVE()s the outer alternative might - * add more padding. To ensure consistent patching find the max - * padding for all alt_instr entries for this site (nested - * alternatives result in consecutive entries). - */ - for (b = a+1; b < end && instr_va(b) == instr_va(a); b++) { - u8 len = max(a->instrlen, b->instrlen); - a->instrlen = b->instrlen = len; - } - - instr = instr_va(a); - replacement = (u8 *)&a->repl_offset + a->repl_offset; - BUG_ON(a->instrlen > sizeof(insn_buff)); - BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32); - - /* - * Patch if either: - * - feature is present - * - feature not present but ALT_FLAG_NOT is set to mean, - * patch if feature is *NOT* present. - */ - if (!boot_cpu_has(a->cpuid) == !(a->flags & ALT_FLAG_NOT)) { - memcpy(insn_buff, instr, a->instrlen); - optimize_nops(instr, insn_buff, a->instrlen); - text_poke_early(instr, insn_buff, a->instrlen); - continue; - } - - DPRINTK(ALT, "feat: %d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d) flags: 0x%x", - a->cpuid >> 5, - a->cpuid & 0x1f, - instr, instr, a->instrlen, - replacement, a->replacementlen, a->flags); - - memcpy(insn_buff, replacement, a->replacementlen); - insn_buff_sz = a->replacementlen; - - if (a->flags & ALT_FLAG_DIRECT_CALL) - insn_buff_sz = alt_replace_call(instr, insn_buff, a); - - for (; insn_buff_sz < a->instrlen; insn_buff_sz++) - insn_buff[insn_buff_sz] = 0x90; - - text_poke_apply_relocation(insn_buff, instr, a->instrlen, replacement, a->replacementlen); + struct patch_site ps; - DUMP_BYTES(ALT, instr, a->instrlen, "%px: old_insn: ", instr); - DUMP_BYTES(ALT, replacement, a->replacementlen, "%px: rpl_insn: ", replacement); - DUMP_BYTES(ALT, insn_buff, insn_buff_sz, "%px: final_insn: ", instr); + analyze_patch_site(&ps, a, end); - text_poke_early(instr, insn_buff, insn_buff_sz); + optimize_nops(ps.instr, ps.buff, ps.len); + text_poke_early(ps.instr, ps.buff, ps.len); } kasan_enable_current(); -- 2.51.0