From: Alex Caudill <alex.caudill@gmail.com>
To: musl@lists.openwall.com
Subject: Re: PATCH: dl_iterate_phdr()
Date: Fri, 12 Oct 2012 20:04:24 -0500 [thread overview]
Message-ID: <CAFXnQt6XPQhDU2-BOwS5Z9mgFcKWY7REJa-jUKA+QYbzGrDs5w@mail.gmail.com> (raw)
In-Reply-To: <20121012154354.GE254@brightrain.aerifal.cx>
[-- Attachment #1: Type: text/plain, Size: 6568 bytes --]
Thanks again, Rich - I think this turned out to be a pretty clean
implementation. I got frustrated with the static version (it's almost
twice as big!), but I'm planning to get it done tomorrow. For now,
though, here's the updated dynamic version and a revised header.
One minor thing: I believe the (Addr)current->base cast is necessary
to ensure correctness (per your previous mail). Please correct me if
I'm wrong.
On 10/12/12, Rich Felker <dalias@aerifal.cx> wrote:
> On Fri, Oct 12, 2012 at 08:28:59AM -0500, Alex Caudill wrote:
>> Thanks for the review! To clarify, I meant that the patch is public
>> domain. This version implements most of your changes and attempts
>> support for dlopen()'d libraries and vdso.
>>
>> Some questions:
>>
>> 1.) I guarded link.h with _GNU_SOURCE or _BSD_SOURCE; I'm unclear on
>> whether this is necessary since it isn't a standard header, so please
>> let me know your preference.
>
> Not needed. I'd rather avoid excessive feature test macro checks like
> this. The only time it might be desirable is when the nonstandard
> header has a strong traditional precedent and the GNU version pollutes
> the namespace with a lot of junk that might break apps. Then, the GNU
> part should be protected.
>
> Also, FYI, any header that checks feature test macros needs to be
> including <features.h> to setup the default features if none are
> explicitly defined.
>
>> 2.) I'm unsure of how to replace ElfW(Phdr) in dl_phdr_info - or did
>> you mean that musl should internally use some opaque type for
>> dl_phdr_info? Also, should ElfW() go in elf.h?
>
> Indeed, I think it's best to just keep using ElfW like you originally
> did, then.
>
> The issue with the types in elf.h is that they are designed for
> writing a program that processes arbitrary (possibly foreign arch) ELF
> files, not for writing native code, and thereby they make it so that
> you have to use ugly macros to get the right native types. Most of
> musl's dynamic linker just uses size_t for the address/word-size
> types, and uint32_t and uint16_t for the unconditionally 32/16-bit
> types, rather than the complex ELF type mess. However, since link.h is
> a public interface, it's probably best to just follow the ugly
> precedent there. After all, if somebody takes &foo->dlpi_addr, they
> expect it to have the right type, and this could break if the ELF type
> is unsigned long while the uintptr_t type is unsigned int (even if
> they have the same size/representation).
>
> Sorry for wasting your time with this issue.
>
>> 3.) The attached dltest.c reveals a problem with dlopen()'d libraries
>> which seems to be related to the locking strategy (see output below).
>> If I don't take the lock and check for current == saved_tail, it
>> "fixes" the example. At the least, I think this reveals a flaw with
>> dlopen("foo", RTLD_NOW) - shouldn't it hold the lock until the dso
>> list has been updated?
>>
>> This function is used by libunwind and (I think) libgcc_eh for C++
>> exception support, and it's possible that additional fields in
>> dl_phdr_info will be necessary in order for those to work unmodified
>> with musl. I'll look into this today and come up with more tests.
>> Solaris and FreeBSD, at least, have these appended to struct
>> dl_phdr_info:
>>
>> unsigned long long int dlpi_adds; /* total # of loads */
>> unsigned long long int dlpi_subs; /* total # of unloads */
>>
>> The dl_iterate_phdr() callback is passed a size arg, and the callback
>> is responsible for checking the size to ensure that the additional
>> fields are present. Don't shoot the messenger! :)
>
> That's no problem; they look easy to add. Also, the last two fields
> for TLS stuff would be easy to add; they're already in struct dso (as
> tls_image and tls_id).
>
>> @@ -57,6 +58,8 @@ struct dso {
>> size_t *dynv;
>> struct dso *next, *prev;
>>
>> + Phdr *phdr;
>> + uint16_t phnum;
>
> It looks like your patch is mixing tabs and spaces. Please use tabs
> for indention, and make indention consistent with the rest of musl.
>
>> if (aux[AT_PHDR]) {
>> size_t interp_off = 0;
>> size_t tls_image = 0;
>> /* Find load address of the main program, via AT_PHDR vs PT_PHDR. */
>> - phdr = (void *)aux[AT_PHDR];
>> - for (i=aux[AT_PHNUM]; i; i--, phdr=(void *)((char *)phdr +
>> aux[AT_PHENT])) {
>> + app->phdr = phdr = (void *)aux[AT_PHDR];
>> + app->phnum = aux[AT_PHNUM];
>> + for (i=app->phnum; i; i--, phdr=(void *)((char *)phdr + aux[AT_PHENT]))
>> {
>
> I would avoid changing the for look here. Hopefully the compiler
> generates the same code either way, but in principle it's
> easier/faster to read from aux[AT_PHNUM] (a fixed offset from the
> stack pointer) than through indirection via app->phnum. And avoiding
> the extra change makes it clear that the patch is not changing much,
> just saving new values.
>
>> +int dl_iterate_phdr(int(*callback)(struct dl_phdr_info *info, size_t
>> size, void *data), void *data)
>> +{
>> + struct dso *current, *saved_tail;
>> + struct dl_phdr_info info;
>> + int ret = 0;
>> +
>> + pthread_rwlock_rdlock(&lock);
>> + saved_tail = tail;
>> + pthread_rwlock_unlock(&lock);
>> +
>> + for(current = head; ; current = current->next) {
>> + info.dlpi_addr = (uintptr_t)current->base;
>> + info.dlpi_name = current->name;
>> + info.dlpi_phdr = current->phdr;
>> + info.dlpi_phnum = current->phnum;
>> +
>> + ret = (callback)(&info, sizeof (info), data);
>> + if (ret != 0) break;
>> +
>> + if (current == saved_tail) break;
>> + }
>> + return ret;
>> +}
>> #else
>> void *dlopen(const char *file, int mode)
>
> After the #else, there are no-op/dummy versions of all the dl
> functions for static linked programs. I think it should be possible to
> do this for dl_iterate_phdr too -- just use __libc.auxv to get the
> AT_PHDR and AT_PHNUM entries, and there's only a single DSO, the main
> program.
>
> Actually with vsyscall, there's also code running from the vdso, but
> exceptions cannot occur in that code so only the debugger needs to be
> aware of its existence. If you want, you could report it with
> __libc.auxv using AT_SYSINFO_EHDR to get the phdr pointer.
>
>> #ifdef _LP64
>
> This should still be something like #if UINTPTR_MAX > 0xffffffff
>
>> struct dl_phdr_info {
>> uintptr_t dlpi_addr;
>> const char *dlpi_name;
>> const ElfW(Phdr) *dlpi_phdr;
>> uint16_t dlpi_phnum;
>> };
>
> And could you use consistent indention (tabs) here?
>
> Rich
>
[-- Attachment #2: dynlink.patch --]
[-- Type: application/octet-stream, Size: 4199 bytes --]
diff --git a/src/ldso/dynlink.c b/src/ldso/dynlink.c
index c3cb611..2802c21 100644
--- a/src/ldso/dynlink.c
+++ b/src/ldso/dynlink.c
@@ -13,6 +13,7 @@
#include <errno.h>
#include <limits.h>
#include <elf.h>
+#include <link.h>
#include <setjmp.h>
#include <pthread.h>
#include <ctype.h>
@@ -30,12 +31,14 @@ static char errbuf[128];
typedef Elf32_Ehdr Ehdr;
typedef Elf32_Phdr Phdr;
typedef Elf32_Sym Sym;
+typedef Elf32_Addr Addr;
#define R_TYPE(x) ((x)&255)
#define R_SYM(x) ((x)>>8)
#else
typedef Elf64_Ehdr Ehdr;
typedef Elf64_Phdr Phdr;
typedef Elf64_Sym Sym;
+typedef Elf64_Addr Addr;
#define R_TYPE(x) ((x)&0xffffffff)
#define R_SYM(x) ((x)>>32)
#endif
@@ -57,6 +60,8 @@ struct dso {
size_t *dynv;
struct dso *next, *prev;
+ Phdr *phdr;
+ uint16_t phnum;
int refcnt;
Sym *syms;
uint32_t *hashtab;
@@ -92,6 +97,7 @@ void *__install_initial_tls(void *);
static struct dso *head, *tail, *libc, *fini_head;
static char *env_path, *sys_path, *r_path;
+static unsigned long long gencnt = 0;
static int ssp_used;
static int runtime;
static int ldd_mode;
@@ -324,6 +330,8 @@ static void *map_library(int fd, struct dso *dso)
eh->e_phoff = sizeof *eh;
}
ph = (void *)((char *)buf + eh->e_phoff);
+ dso->phdr = ph;
+ dso->phnum = (uint16_t)eh->e_phnum;
for (i=eh->e_phnum; i; i--, ph=(void *)((char *)ph+eh->e_phentsize)) {
if (ph->p_type == PT_DYNAMIC)
dyn = ph->p_vaddr;
@@ -815,18 +823,19 @@ void *__dynlink(int argc, char **argv)
lib->name = lib->shortname = "libc.so";
lib->global = 1;
ehdr = (void *)lib->base;
- find_map_range((void *)(aux[AT_BASE]+ehdr->e_phoff),
- ehdr->e_phnum, ehdr->e_phentsize, lib);
- lib->dynv = (void *)(lib->base + find_dyn(
- (void *)(aux[AT_BASE]+ehdr->e_phoff),
- ehdr->e_phnum, ehdr->e_phentsize));
+ lib->phnum = (uint16_t)ehdr->e_phnum;
+ lib->phdr = (void *)(aux[AT_BASE]+ehdr->e_phoff);
+ find_map_range(lib->phdr, ehdr->e_phnum, ehdr->e_phentsize, lib);
+ lib->dynv = (void *)(lib->base + find_dyn(lib->phdr,
+ ehdr->e_phnum, ehdr->e_phentsize));
decode_dyn(lib);
if (aux[AT_PHDR]) {
size_t interp_off = 0;
size_t tls_image = 0;
/* Find load address of the main program, via AT_PHDR vs PT_PHDR. */
- phdr = (void *)aux[AT_PHDR];
+ app->phdr = phdr = (void *)aux[AT_PHDR];
+ app->phnum = aux[AT_PHNUM];
for (i=aux[AT_PHNUM]; i; i--, phdr=(void *)((char *)phdr + aux[AT_PHENT])) {
if (phdr->p_type == PT_PHDR)
app->base = (void *)(aux[AT_PHDR] - phdr->p_vaddr);
@@ -890,7 +899,8 @@ void *__dynlink(int argc, char **argv)
/* Attach to vdso, if provided by the kernel */
if (search_vec(auxv, &vdso_base, AT_SYSINFO_EHDR)) {
ehdr = (void *)vdso_base;
- phdr = (void *)(vdso_base + ehdr->e_phoff);
+ vdso->phdr = phdr = (void *)(vdso_base + ehdr->e_phoff);
+ vdso->phnum = ehdr->e_phnum;
for (i=ehdr->e_phnum; i; i--, phdr=(void *)((char *)phdr + ehdr->e_phentsize)) {
if (phdr->p_type == PT_DYNAMIC)
vdso->dynv = (void *)(vdso_base + phdr->p_offset);
@@ -1051,6 +1061,7 @@ void *dlopen(const char *file, int mode)
orig_tail = tail;
end:
__release_ptc();
+ if (errflag == 0) gencnt++;
pthread_rwlock_unlock(&lock);
if (p) do_init_fini(orig_tail);
pthread_setcancelstate(cs, 0);
@@ -1166,6 +1177,33 @@ void *__dlsym(void *restrict p, const char *restrict s, void *restrict ra)
pthread_rwlock_unlock(&lock);
return res;
}
+
+int dl_iterate_phdr(int(*callback)(struct dl_phdr_info *info, size_t size, void *data), void *data)
+{
+ struct dso *current;
+ struct dl_phdr_info info;
+ int ret = 0;
+ for(current = head; current;) {
+ info.dlpi_addr = (Addr)current->base;
+ info.dlpi_name = current->name;
+ info.dlpi_phdr = current->phdr;
+ info.dlpi_phnum = current->phnum;
+ info.dlpi_adds = gencnt;
+ info.dlpi_subs = 0;
+ info.dlpi_tls_modid = current->tls_id;
+ info.dlpi_tls_data = current->tls_image;
+
+ ret = (callback)(&info, sizeof (info), data);
+
+ if (ret != 0) break;
+ if (current == tail) break;
+
+ pthread_rwlock_rdlock(&lock);
+ current = current->next;
+ pthread_rwlock_unlock(&lock);
+ }
+ return ret;
+}
#else
void *dlopen(const char *file, int mode)
{
[-- Attachment #3: link.h --]
[-- Type: text/x-chdr, Size: 539 bytes --]
#ifndef _LINK_H
#define _LINK_H
#include <elf.h>
#define __NEED_size_t
#include <bits/alltypes.h>
#if UINTPTR_MAX > 0xffffffff
#define ElfW(type) Elf64_ ## type
#else
#define ElfW(type) Elf32_ ## type
#endif
struct dl_phdr_info {
ElfW(Addr) dlpi_addr;
const char *dlpi_name;
const ElfW(Phdr) *dlpi_phdr;
ElfW(Half) dlpi_phnum;
unsigned long long int dlpi_adds;
unsigned long long int dlpi_subs;
size_t dlpi_tls_modid;
void *dlpi_tls_data;
};
int dl_iterate_phdr(int (*)(struct dl_phdr_info *, size_t, void *), void *);
#endif
next prev parent reply other threads:[~2012-10-13 1:04 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-11 14:29 Alex Caudill
2012-10-11 23:42 ` Rich Felker
2012-10-12 0:00 ` Rich Felker
2012-10-12 13:28 ` Alex Caudill
2012-10-12 15:43 ` Rich Felker
2012-10-13 1:04 ` Alex Caudill [this message]
2012-10-13 1:24 ` Alex Caudill
2012-10-13 1:31 ` Rich Felker
2012-10-15 3:30 ` Rich Felker
2012-10-15 4:47 ` Alex Caudill
2012-10-15 12:41 ` Rich Felker
2012-10-18 20:45 ` Rich Felker
2012-10-31 18:35 ` Alex Caudill
2012-11-01 1:33 ` Rich Felker
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