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Make hwpmc work for userspace binaries again

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hwpmc has been utterly broken for userspace binaries, and has been
labeling all samples from userspace binaries as dubious frames. The
issues are that:

-The check for ph.p_offset & (-ph.p_align) == 0 was mostly bogus. The
 intent was to ignore all executable segments other than the first,
 which when using BFD appeared in the first page, but with current LLD
 a read-only data segment appears before the executable segment,
 pushing the latter into the second page or later. This meant no
 executable segment was ever found, and thus pi_vaddr remained
 0. Instead of relying on BFD's layout, track whether we've seen an
 executable segment explicitly with a local bool.

-Shared libraries were not parsing the segments to calculate pi_vaddr,
 resulting in it always being 0. Again, when using BFD, the executable
 segment started at the first page, and so pi_vaddr was genuinely
 meant to be 0, but not with LLD's current layout. This meant that
 pmcstat_image_link's offset calculation gave the base address of the
 segment in memory, rather than the base address of the whole library
 in memory, and so when adding that to pi_start/pi_end to get the
 range of the executable sections in memory it double-counted the
 offset of the first executable segment within the library. Thus we
 need to do the exact same parsing for ET_DYN as we do for ET_EXEC,
 which is simpler to write as special-casing ET_REL to not look for
 segments. Note that, whilst PT_INTERP isn't needed for shared
 libraries, it will be for PIEs, which pmcstat still fails to handle
 due to not knowing the base address of the PIE; we get the base
 address for libraries by MAP_IN events, and for rtld by virtue of the
 process's entry address being rtld's, but have no equivalent for the
 executable.

Fixes courtesy of jrtc27@.

Reviewed by: jrtc27, jhb (earlier version)
Differential Revision: https://reviews.freebsd.org/D33055
Sponsored by: Netflix
This commit is contained in:
Andrew Gallatin 2021-12-15 08:38:36 -05:00
parent cec04a5d71
commit 517a7adb11
1 changed files with 7 additions and 2 deletions
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9
lib/libpmcstat/libpmcstat_image.c
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@ -43,6 +43,7 @@ __FBSDID("$FreeBSD$");
#include <fcntl.h>
#include <pmc.h>
#include <pmclog.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
@ -295,6 +296,7 @@ pmcstat_image_get_elf_params(struct pmcstat_image *image,
size_t i, nph, nsh;
const char *path, *elfbase;
char *p, *endp;
bool first_exec_segment;
uintfptr_t minva, maxva;
Elf *e;
Elf_Scn *scn;
@ -384,7 +386,7 @@ pmcstat_image_get_elf_params(struct pmcstat_image *image,
* loaded. Additionally, for dynamically linked executables,
* save the pathname to the runtime linker.
*/
if (eh.e_type == ET_EXEC) {
if (eh.e_type != ET_REL) {
if (elf_getphnum(e, &nph) == 0) {
warnx(
"WARNING: Could not determine the number of program headers in \"%s\": %s.",
@ -392,6 +394,7 @@ pmcstat_image_get_elf_params(struct pmcstat_image *image,
elf_errmsg(-1));
goto done;
}
first_exec_segment = true;
for (i = 0; i < eh.e_phnum; i++) {
if (gelf_getphdr(e, i, &ph) != &ph) {
warnx(
@ -416,8 +419,10 @@ pmcstat_image_get_elf_params(struct pmcstat_image *image,
break;
case PT_LOAD:
if ((ph.p_flags & PF_X) != 0 &&
(ph.p_offset & (-ph.p_align)) == 0)
first_exec_segment) {
image->pi_vaddr = ph.p_vaddr & (-ph.p_align);
first_exec_segment = false;
}
break;
}
}