[PATCH] kexec: derive purgatory entry from symbol

[Li Chen <me@linux.beauty>]

kexec_load_purgatory() derives image->start by locating e_entry inside an
SHF_EXECINSTR section. If the purgatory object contains multiple executable
sections with overlapping sh_addr, the entrypoint check can match more than
once and trigger a WARN.

Derive the entry section from the purgatory_start symbol when present and
compute image->start from its final placement. Keep the existing e_entry
fallback for purgatories that do not expose the symbol.

WARNING: kernel/kexec_file.c:1009 at kexec_load_purgatory+0x395/0x3c0, CPU#10: kexec/1784
Call Trace:
 <TASK>
 bzImage64_load+0x133/0xa00
 __do_sys_kexec_file_load+0x2b3/0x5c0
 do_syscall_64+0x81/0x610
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fixes: 8652d44f466a ("kexec: support purgatories with .text.hot sections")
Signed-off-by: Li Chen <me@linux.beauty>
---
 kernel/kexec_file.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 3f1d6c4e8ff2..d3c68c6b4bbc 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -880,6 +880,8 @@ static int kexec_calculate_store_digests(struct kimage *image)
 }
 
 #ifdef CONFIG_ARCH_SUPPORTS_KEXEC_PURGATORY
+static const Elf_Sym * kexec_purgatory_find_symbol(struct purgatory_info *pi,
+			      const char *name);
 /*
  * kexec_purgatory_setup_kbuf - prepare buffer to load purgatory.
  * @pi:		Purgatory to be loaded.
@@ -958,6 +960,10 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
 	unsigned long offset;
 	size_t sechdrs_size;
 	Elf_Shdr *sechdrs;
+	const Elf_Sym *entry_sym;
+	u16 entry_shndx = 0;
+	unsigned long entry_off = 0;
+	bool start_fixed = false;
 	int i;
 
 	/*
@@ -975,6 +981,12 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
 	bss_addr = kbuf->mem + kbuf->bufsz;
 	kbuf->image->start = pi->ehdr->e_entry;
 
+	entry_sym = kexec_purgatory_find_symbol(pi, "purgatory_start");
+	if (entry_sym) {
+		entry_shndx = entry_sym->st_shndx;
+		entry_off = entry_sym->st_value;
+	}
+
 	for (i = 0; i < pi->ehdr->e_shnum; i++) {
 		unsigned long align;
 		void *src, *dst;
@@ -992,6 +1004,13 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
 
 		offset = ALIGN(offset, align);
 
+		if (!start_fixed && entry_sym && i == entry_shndx &&
+		    (sechdrs[i].sh_flags & SHF_EXECINSTR) &&
+		    entry_off < sechdrs[i].sh_size) {
+			kbuf->image->start = kbuf->mem + offset + entry_off;
+			start_fixed = true;
+		}
+
 		/*
 		 * Check if the segment contains the entry point, if so,
 		 * calculate the value of image->start based on it.
@@ -1002,13 +1021,14 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
 		 * is not set to the initial value, and warn the user so they
 		 * have a chance to fix their purgatory's linker script.
 		 */
-		if (sechdrs[i].sh_flags & SHF_EXECINSTR &&
+		if (!start_fixed && sechdrs[i].sh_flags & SHF_EXECINSTR &&
 		    pi->ehdr->e_entry >= sechdrs[i].sh_addr &&
 		    pi->ehdr->e_entry < (sechdrs[i].sh_addr
 					 + sechdrs[i].sh_size) &&
-		    !WARN_ON(kbuf->image->start != pi->ehdr->e_entry)) {
+		    kbuf->image->start == pi->ehdr->e_entry) {
 			kbuf->image->start -= sechdrs[i].sh_addr;
 			kbuf->image->start += kbuf->mem + offset;
+			start_fixed = true;
 		}
 
 		src = (void *)pi->ehdr + sechdrs[i].sh_offset;
-- 
2.52.0